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This change basically ported our target multilib to the host side.
It supports 2 host build modes: x86 and x86_64 multilib build.
For now you need to set "BUILD_HOST_64bit=true" to switch to x86_64
multilib build. Later we'll default to x86_64 build and have a flag
to force 32-bit only build, which may be needed by SDK build.

In host module definition, like in target ones, you can use the
following
LOCAL variables to set up multilib configuration:
LOCAL_MULTILIB: can be "both", "first", "32" or "64".
It also supports the same set of arch or 32-vs-64 specific LOCAL
variables.
By default, it builds only for the first arch.

To keep path compatibility, in x86_64 build files are still output to
out/host/linux-x86; Both 32-bit and 64-bit executables are in
out/host/linux-86/bin;
In x86_64 build 32-bit shared libraries are installed to
out/host/linux-x86/lib32
and 64-bit shared libraries are installed to out/host/linux-x86/lib;
32-bit object files are output to out/host/linux-x86/ob...