runtime: make sync_atomic_SwapPointer signature match sync/atomic

SwapPointer is declared as

  func SwapPointer(addr *unsafe.Pointer, new unsafe.Pointer) (old unsafe.Pointer)

in sync/atomic, but defined in the runtime (where it's actually
implemented) as

  func sync_atomic_SwapPointer(ptr unsafe.Pointer, new unsafe.Pointer) unsafe.Pointer

Make ptr a *unsafe.Pointer in the runtime definition to match the type
in sync/atomic.

Change-Id: I99bab651b995001bbe54f9e790fdef2417ef0e9e
Reviewed-on: https://go-review.googlesource.com/21998
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/runtime/atomic_pointer.go

@@ -53,9 +53,9 @@ func sync_atomic_SwapUintptr(ptr *uintptr, new uintptr) uintptr
 
 //go:linkname sync_atomic_SwapPointer sync/atomic.SwapPointer
 //go:nosplit
-func sync_atomic_SwapPointer(ptr unsafe.Pointer, new unsafe.Pointer) unsafe.Pointer {
-	old := unsafe.Pointer(sync_atomic_SwapUintptr((*uintptr)(noescape(ptr)), uintptr(new)))
-	writebarrierptr_nostore((*uintptr)(ptr), uintptr(new))
+func sync_atomic_SwapPointer(ptr *unsafe.Pointer, new unsafe.Pointer) unsafe.Pointer {
+	old := unsafe.Pointer(sync_atomic_SwapUintptr((*uintptr)(noescape(unsafe.Pointer(ptr))), uintptr(new)))
+	writebarrierptr_nostore((*uintptr)(unsafe.Pointer(ptr)), uintptr(new))
 	return old
 }