Enable x86 support for Unix

Also enabled cross compilation of x86 binaries on x64 host.

Ubuntu Dockerfiles:

<details>
  <summary><b><ins>Baseline: x64 build on x64 host</ins></b></summary>

```dockerfile
FROM ubuntu
RUN cat /etc/*-release

RUN apt-get update
RUN apt-get install -y \
        autoconf bash clang cmake gcc libtool curl \
        libunwind-dev llvm make openssl lldb git uuid-dev

RUN git clone https://github.com/dotnet/corert -b master --single-branch

WORKDIR /corert

RUN ./build.sh          # or ./build.sh x64

```
</details>

<details>
  <summary><b><ins>PR: x86 build on x86 host</ins></b></summary>

```dockerfile
FROM i386/ubuntu
RUN cat /etc/*-release

RUN apt-get update
RUN apt-get install -y \
        bash clang cmake gcc libtool curl \
        libunwind-dev llvm make openssl lldb git uuid-dev

RUN git clone https://github.com/am11/corert -b linux-x86 --single-branch

WORKDIR /corert

RUN ./build.sh x86
```
</details>

<details>
  <summary><b><ins>PR: x86 build on x64 host</ins></b></summary>

```dockerfile
FROM ubuntu
RUN cat /etc/*-release

RUN apt-get update
RUN apt-get install -y \
        bash clang cmake gcc libtool curl \
        libunwind-dev llvm make openssl lldb git uuid-dev \
        g++-multilib

RUN git clone https://github.com/am11/corert -b linux-x86 --single-branch

WORKDIR /corert

RUN ./build.sh x86
```
</details>

10 files changed, 50 insertions, 0 deletions

diff --git a/src/Native/Runtime/i386/AllocFast.S b/src/Native/Runtime/i386/AllocFast.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/AllocFast.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/CallDescrWorker.S b/src/Native/Runtime/i386/CallDescrWorker.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/CallDescrWorker.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/CallingConventionConverterHelpers.S b/src/Native/Runtime/i386/CallingConventionConverterHelpers.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/CallingConventionConverterHelpers.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/ExceptionHandling.S b/src/Native/Runtime/i386/ExceptionHandling.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/ExceptionHandling.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/Interlocked.S b/src/Native/Runtime/i386/Interlocked.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/Interlocked.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/InteropThunksHelpers.S b/src/Native/Runtime/i386/InteropThunksHelpers.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/InteropThunksHelpers.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/PInvoke.S b/src/Native/Runtime/i386/PInvoke.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/PInvoke.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/StubDispatch.S b/src/Native/Runtime/i386/StubDispatch.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/StubDispatch.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/UniversalTransition.S b/src/Native/Runtime/i386/UniversalTransition.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/UniversalTransition.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement
diff --git a/src/Native/Runtime/i386/WriteBarriers.S b/src/Native/Runtime/i386/WriteBarriers.S
new file mode 100644
index 000000000..1dc275673
--- /dev/null
+++ b/src/Native/Runtime/i386/WriteBarriers.S
@@ -0,0 +1,5 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// TODO: Implement