Switch unrolled loop in BN_usub with memcpy.

See also upstream's 06cf881a3a10d5af3c1255c08cfd0c6ddb5f1cc3,
9f040d6decca7930e978784c917f731e5c45e8f0, and
9f6795e7d2d1e35668ad70ba0afc480062be4e2e.

Change-Id: I27d90e382867a5fe988d152b31f8494e001a6a9f
Reviewed-on: https://boringssl-review.googlesource.com/6628
Reviewed-by: Adam Langley <agl@google.com>

crypto/bn/add.c

@@ -56,6 +56,8 @@
 
 #include <openssl/bn.h>
 
+#include <string.h>
+
 #include <openssl/err.h>
 #include <openssl/mem.h>
 
@@ -311,27 +313,8 @@ int BN_usub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b) {
     }
   }
 
-  if (rp != ap) {
+  if (dif > 0 && rp != ap) {
-    for (;;) {
+    memcpy(rp, ap, sizeof(*rp) * dif);
-      if (!dif--) {
-        break;
-      }
-      rp[0] = ap[0];
-      if (!dif--) {
-        break;
-      }
-      rp[1] = ap[1];
-      if (!dif--) {
-        break;
-      }
-      rp[2] = ap[2];
-      if (!dif--) {
-        break;
-      }
-      rp[3] = ap[3];
-      rp += 4;
-      ap += 4;
-    }
   }
 
   r->top = max;