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>
