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>

3 files changed, 118 insertions, 0 deletions

diff --git a/src/Native/inc/unix/poppack.h b/src/Native/inc/unix/poppack.h
new file mode 100644
index 000000000..aee3b49a3
--- /dev/null
+++ b/src/Native/inc/unix/poppack.h
@@ -0,0 +1,40 @@
+// 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.
+//
+
+//
+// ===========================================================================
+// File:  poppack.h
+//
+// ===========================================================================
+/*
+Abstract:
+
+    This file turns packing of structures off.  (That is, it enables
+    automatic alignment of structure fields.)  An include file is needed
+    because various compilers do this in different ways.
+
+    poppack.h is the complement to pshpack?.h.  An inclusion of poppack.h
+    MUST ALWAYS be preceded by an inclusion of one of pshpack?.h, in one-to-one
+    correspondence.
+
+    For Microsoft compatible compilers, this file uses the pop option
+    to the pack pragma so that it can restore the previous saved by the
+    pshpack?.h include file.
+
+*/
+
+#if ! (defined(lint) || defined(RC_INVOKED))
+#if ( _MSC_VER >= 800 && !defined(_M_I86)) || defined(_PUSHPOP_SUPPORTED)
+#pragma warning(disable:4103)
+#if !(defined( MIDL_PASS )) || defined( __midl )
+#pragma pack(pop)
+#else
+#pragma pack()
+#endif
+#else
+#pragma pack()
+#endif
+#endif // ! (defined(lint) || defined(RC_INVOKED))
+
diff --git a/src/Native/inc/unix/pshpack1.h b/src/Native/inc/unix/pshpack1.h
new file mode 100644
index 000000000..884e4c9c4
--- /dev/null
+++ b/src/Native/inc/unix/pshpack1.h
@@ -0,0 +1,39 @@
+// 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.
+//
+
+//
+// ===========================================================================
+// File: pshpack1.h
+//
+// ===========================================================================
+
+/*++
+
+Abstract:
+
+    This file turns 1 byte packing of structures on.  (That is, it disables
+    automatic alignment of structure fields.)  An include file is needed
+    because various compilers do this in different ways.  For Microsoft
+    compatible compilers, this files uses the push option to the pack pragma
+    so that the poppack.h include file can restore the previous packing
+    reliably.
+
+    The file poppack.h is the complement to this file.
+
+--*/
+
+#if ! (defined(lint) || defined(RC_INVOKED))
+#if ( _MSC_VER >= 800 && !defined(_M_I86)) || defined(_PUSHPOP_SUPPORTED)
+#pragma warning(disable:4103)
+#if !(defined( MIDL_PASS )) || defined( __midl )
+#pragma pack(push,1)
+#else
+#pragma pack(1)
+#endif
+#else
+#pragma pack(1)
+#endif
+#endif // ! (defined(lint) || defined(RC_INVOKED))
+
diff --git a/src/Native/inc/unix/pshpack4.h b/src/Native/inc/unix/pshpack4.h
new file mode 100644
index 000000000..c72dec96e
--- /dev/null
+++ b/src/Native/inc/unix/pshpack4.h
@@ -0,0 +1,39 @@
+// 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.
+//
+
+//
+// ===========================================================================
+// File: pshpack4.h
+//
+// ===========================================================================
+
+/*++
+
+Abstract:
+
+    This file turns 4 byte packing of structures on.  (That is, it disables
+    automatic alignment of structure fields.)  An include file is needed
+    because various compilers do this in different ways.  For Microsoft
+    compatible compilers, this files uses the push option to the pack pragma
+    so that the poppack.h include file can restore the previous packing
+    reliably.
+
+    The file poppack.h is the complement to this file.
+
+--*/
+
+#if ! (defined(lint) || defined(RC_INVOKED))
+#if ( _MSC_VER >= 800 && !defined(_M_I86)) || defined(_PUSHPOP_SUPPORTED)
+#pragma warning(disable:4103)
+#if !(defined( MIDL_PASS )) || defined( __midl )
+#pragma pack(push,4)
+#else
+#pragma pack(4)
+#endif
+#else
+#pragma pack(4)
+#endif
+#endif // ! (defined(lint) || defined(RC_INVOKED))
+