[libcxxabi][libunwind] Support for using LLVM libc (#134893)

This generalizes the support added in #99287 renaming the option to
RUNTIMES_USE_LIBC and integrating the module into libc++abi and
libunwind as well.

NOKEYCHECK=True
GitOrigin-RevId: 9d3b60dea7ad81f6cb0d9f2e51bb6c80c03fa2d4
2 files changed