Currently, llvm-ml only assembles files that have the .asm extension, so
push_registers_masm.S fails to get assembled. This CL changes the
extension of the x86 and x64 push_registers_masm.S files to .asm.

I'll work on a patch to support assembling files with the .S extension
in llvm-ml, but in the meantime, we should probably rename the files as
it is customary for Intel syntax assembly files to have the .asm
extension[0].

ARM assembly files don't use llvm-ml, so we don't need to rename them.

[0]: https://stackoverflow.com/a/34098830

Change-Id: Ie4db42b78dd358c3ec0de83e9518aa5c60f6d175
Bug: chromium:762167
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3668287
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80772}