armclang: add empty sys/cdefs.h

Various headers include <sys/cdefs.h> like zephyr/posix/sys/stat.h.
When building with the arm-clang toolchain we utilize armstdc
so we need an empty cdefs.h similar to what minimal libc has so
things will build.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>