build:zos: updates to use ibm-clang

Enable V8 to build on z/OS using IBM Open XL C/C++ for z/OS compiler,
available from https://www.ibm.com/products/xl-cpp-compiler-zos.

Changes include defines, cflags, asmflags, cc, cxx, etc.

Bug: b:350938993
Change-Id: I7c410092dad9c1a6ce26dffb46d769e7addbdb39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5678523
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1324430}
NOKEYCHECK=True
GitOrigin-RevId: 3619bab6464aec622f86daba8efcf4f0046c2ebf
2 files changed