[llvm] [bazel] Improve building on/for Windows (PR #171761)

[David Zbarsky via llvm-commits]

dzbarsky wrote:

> > Although...should we just use windows_gnu.S for now? I missed the existence of it
> 
> Maybe... but I don't have a windows env for testing. Cc @mstorsjo in case you need help.

I don't have an easily acceissble windows env either but I have a nice setup where I can cross-compile a windows-targeted LLVM from linux and then use windows-target llvm on windows host in CI to cross-build a linux binary :) Do you think its enough to do that with `build-id=md5`?

https://github.com/llvm/llvm-project/pull/171761