[libcxx] [llvm] [libc++] Add support for picolibc and newlib in RUNTIMES_USE_LIBC (PR #147956)

[Volodymyr Turanskyy via llvm-commits]

voltur01 wrote:

Thanks, I will double check if we can tidy up more of picolibc checks - the new defines are available in libcxx only, though.

https://github.com/llvm/llvm-project/pull/147956