Allow the compiler to use __builtin_memcpy

In particular this allows the compiler to avoid heavy-handed machinery
if the to be copied buffer is small.

Reviewed by:	jhb

sys/sys/systm.h

@@ -275,6 +275,7 @@ void	bzero(void * _Nonnull buf, size_t len);
 void	explicit_bzero(void * _Nonnull, size_t);
 
 void	*memcpy(void * _Nonnull to, const void * _Nonnull from, size_t len);
+#define memcpy(to, from, len) __builtin_memcpy(to, from, len)
 void	*memmove(void * _Nonnull dest, const void * _Nonnull src, size_t n);
 
 int	copystr(const void * _Nonnull __restrict kfaddr,

sys/sys/zutil.h

@@ -32,7 +32,6 @@
 #include <sys/param.h>
 #include <sys/kernel.h>
 #  define HAVE_MEMCPY
-#  define memcpy(d, s, n)	bcopy((s), (d), (n))
 #  define memset(d, v, n)	bzero((d), (n))
 #  define memcmp		bcmp