Support building 32-bit C++ binaries on Windows with MSVC toolchain

[mai12]

This PR will allow Bazel to compile 32-bit c++ build targets on windows. A new cc toolchain (cc-compiler-x64_x86_windows) is added to support compiling 32-bit c++ binaries using MSVC compiler.

To use this toolchain, the --cpu option in the build command needs to be set to x64_x86_windows.

Fixes: #10963
@meteorcloudy

[meteorcloudy]

Thanks for working on this! A few suggestions first:

• Can you add a brief description about this PR? What it does, how should user use this new feature?
• When creating a new commit, it's better to also have a more detailed message as the title instead of only "Support compiling 32-bit binaries". It will make it easier to review ;)

[meteorcloudy]

This is real nice implementation! Thanks!
Can you add the test? 😃

[meteorcloudy]

bazel/tools/cpp/windows_cc_toolchain_config.bzl

Line 501 in 40e7636

flags = ["/MACHINE:X64"],

Another /MACHINE:X64 is hard coded in windows_cc_toolchain_config.bzl, you probably need to refactor it as an attribute like the default_link_flags, otherwise the build will fail with:

bazel-out\x64_x86_windows-fastbuild\bin\examples\cpp\_objs\hello-lib\hello-lib.obj : fatal error LNK1112: module machine type 'x86' conflicts with target machine type 
'x64'
Target //examples/cpp:hello-world failed to build

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[meteorcloudy]

@mai12 It looks like you committed with a different account? Can you switch back to the same account?

[meteorcloudy]

I think you can do a force push to override the commits pushed by another account?

[mai12]

@googlebot I fixed it.

[mai12]

@googlebot I fixed it.

[googlebot]

CLAs look good, thanks!

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