toolchain: llvm: Provide working BUILD_ASSERT macro

The resulting BUILD_ASSERT macro provided by
zephyr/toolchain/gcc.h is defined to be empty, making the macro
useless.

This commit ensures we define it to something that actually
performs a validation. When not compiling for C++ or >=C11,
we rely on that cdefs.h provides _Static_assert.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>