Allow --cpu=x64_arm64_windows as a compilation target on Windows

[ianier]

Compilation of TensorFlow on Windows currently assumes x64 as a target CPU architecture. Searching .bzl files for "x64_windows" shows the many places in TF where this assumption is made.

In principle, though, nothing should prevent compiling TensorFlow to target ARM64 on Windows (--cpu=x64_arm64_windows, see the bazel doc).

This feature request is about adding support for '--cpu=x64_arm64_windows' to produce ARM64 binaries. The goal is to be able to use TF on Windows on ARM 64 bits.