runtime: fix use of xadd64

Commit 7407d8e5 was rebased over the switch to runtime/internal/atomic
and introduced a call to xadd64, which no longer exists. Fix that
call.

Change-Id: I99c93469794c16504ae4a8ffe3066ac382c66a3a
Reviewed-on: https://go-review.googlesource.com/16816Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcsweep.go

@@ -392,7 +392,7 @@ func reimburseSweepCredit(unusableBytes uintptr) {
 		// Nobody cares about the credit. Avoid the atomic.
 		return
 	}
-	xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))
+	atomic.Xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))
 }
 
 func dumpFreeList(s *mspan) {