lib: cpp: Allow selecting libc++ with picolibc

LLVM C++ Standard Library aka. libc++ may be used with Picolibc -- for
example, LLVM Embedded Toolchain for Arm ships with Picolibc and libc++
compiled for Picolibc, and so does Zephyr SDK LLVM toolchain.

Ideally, we would a flag like `TOOLCHAIN_HAS_LIBCXX_PICOLIBC` indicating
that toolchain has libc++ compiled specifically for Picolibc; but, we do
not want to overcomplicate the toolchain feature flags at this time without
proper underlying infrastructure to handle it.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>