tests: libcxx: Add LLVM libc++/picolibc test

This commit adds a new C++ standard library test variant that builds with
Picolibc and LLVM C++ Standard Library aka. libc++ on the supported
toolchains (e.g. LLVM Embedded Toolchain for Arm and Zephyr SDK LLVM).

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