cmd/compile: intrinsify runtime/internal/atomic.Xaddint64

This aliases to Xadd64.

Change-Id: I95d49e1d03eecc242e9e6fd4b2742b1c1a1d5ade
Reviewed-on: https://go-review.googlesource.com/29274
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -2619,6 +2619,8 @@ func intrinsicInit() {
 	// aliases internal to runtime/internal/atomic
 	i.std[intrinsicKey{"runtime/internal/atomic", "Loadint64"}] =
 		i.std[intrinsicKey{"runtime/internal/atomic", "Load64"}]
+	i.std[intrinsicKey{"runtime/internal/atomic", "Xaddint64"}] =
+		i.std[intrinsicKey{"runtime/internal/atomic", "Xadd64"}]
 
 	// intrinsics which vary depending on the size of int/ptr.
 	i.intSized = map[sizedIntrinsicKey]intrinsicBuilder{