[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 commented:

Just a small comment, but LGTM otherwise.

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