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>