runtime: atomic ops for int64

These currently use portable implementations in terms of their uint64
counterparts.

Change-Id: Icba5f7134cfcf9d0429edabcdd73091d97e5e905
Reviewed-on: https://go-review.googlesource.com/8831Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/stubs.go

@@ -151,6 +151,22 @@ func atomicloaduintptr(ptr *uintptr) uintptr
 //go:noescape
 func atomicloaduint(ptr *uint) uint
 
+// TODO: Write native implementations of int64 atomic ops (or improve
+// inliner). These portable ones can't be inlined right now, so we're
+// taking an extra function call hit.
+
+func atomicstoreint64(ptr *int64, new int64) {
+	atomicstore64((*uint64)(unsafe.Pointer(ptr)), uint64(new))
+}
+
+func atomicloadint64(ptr *int64) int64 {
+	return int64(atomicload64((*uint64)(unsafe.Pointer(ptr))))
+}
+
+func xaddint64(ptr *int64, delta int64) int64 {
+	return int64(xadd64((*uint64)(unsafe.Pointer(ptr)), delta))
+}
+
 //go:noescape
 func setcallerpc(argp unsafe.Pointer, pc uintptr)