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

[William Huynh via llvm-commits]

saturn691 wrote:

This PR would be very useful to me as an LLVM-libc contributor, if you could add back support for `_LIBCPP_LIBC_LLVMLIBC ` - I would like a define that I can access through `libcxx/include/__config`- currently the `__LLVM_LIBC__` define only works during compile time (of the toolchain) and doesn't work during compile time (of the user code).

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