This replaces detection of picolibc in libc++ (by checking for and
including picolibc.h) with using RUNTIMES_USE_LIBC build time option
intriduced in https://github.com/llvm/llvm-project/pull/134893
RUNTIMES_USE_LIBC is extended to accept picolibc and newlib.
Detection of picolibc via the header is kept as a deprecated feature to
avoid breaking builds.
libc++ is updated to use dedicated LIBCXX_LIBC_NEWLIB macro to check for
newlib specific conditions instead of less informative _NEWLIB_VERSION
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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>
a6643f27ec