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

[Lucas Duarte Prates via llvm-commits]

https://github.com/pratlucas approved this pull request.

LGTM, but as per the contribution policy this PR probably needs approval from someone from the libc++ review group.

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