stdlib: add memcpy16() to string.h

This patch exports memcpy16() for platforms, as an option to
memcpy().

Change-Id: I5d4e1cfb4608ec3674224b1447fdd740de549b1f
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>

include/lib/stdlib/string.h

@@ -52,6 +52,7 @@ __BEGIN_DECLS
 void	*memchr(const void *, int, size_t) __pure;
 int	 memcmp(const void *, const void *, size_t) __pure;
 void	*memcpy(void * __restrict, const void * __restrict, size_t);
+void	*memcpy16(void * __restrict, const void * __restrict, size_t);
 void	*memmove(void *, const void *, size_t);
 void	*memset(void *, int, size_t);