Refactor `stgReallocForGMP` to use `memcpy`

GCC is able to generate better code when using `memcpy` instead of
manually copying bytes in a loop. Otoh, `stgAllocForGMP` is typically
called for enlarging initial single-limb structures (see also #8647 for
more information) and so this minor optimization won't be very visible
in measurements.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

cbits/alloc.c

@@ -4,6 +4,8 @@
  *
  * ---------------------------------------------------------------------------*/
 
+#include <string.h>
+
 #include "Rts.h"
 
 #include "gmp.h"
@@ -83,19 +85,9 @@ stgAllocForGMP (size_t size_in_bytes)
 void *
 stgReallocForGMP (void *ptr, size_t old_size, size_t new_size)
 {
-    size_t min_size;
-    void *new_stuff_ptr = stgAllocForGMP(new_size);
-    nat i = 0;
-    char *p = (char *) ptr;
-    char *q = (char *) new_stuff_ptr;
-
-    min_size = old_size < new_size ? old_size : new_size;
-    /* TODO: use memcpy */
-    for (; i < min_size; i++, p++, q++) {
-        *q = *p;
-    }
-
-    return(new_stuff_ptr);
+  size_t min_size = old_size < new_size ? old_size : new_size;
+
+  return memcpy(stgAllocForGMP(new_size), ptr, min_size);
 }
 
 void