armstdc: add string.h & strings.h header

Add string.h header that declerates prototype for strnlen()
as this is not provided by the toolchain headers.

Add strings.h to allow anything that expects strncasecmp() to
be defined there to build correctly.  The arm toolchain actually
defines strncasecmp() in <string.h> so we just have <strings.h>
include <string.h>.

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