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 <[email protected]>

Author: stephanosio

lib/cpp/Kconfig

@@ -123,7 +123,7 @@ config LIBCXX_LIBCPP
 	bool "LLVM C++ Standard Library"
 	depends on "$(TOOLCHAIN_HAS_LIBCXX)" = "y"
 	depends on !NATIVE_APPLICATION
-	depends on NEWLIB_LIBC
+	depends on NEWLIB_LIBC || PICOLIBC
 	select FULL_LIBCPP_SUPPORTED
 	help
 	  Build with LLVM C++ Standard Library (libc++) provided by LLVM