runtime: move machport into darwin's mOS

It's not needed on other OSes.

Change-Id: Ia6b13510585392a7062374806527d33876beba2a
Reviewed-on: https://go-review.googlesource.com/19818
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/os_darwin.go

@@ -7,6 +7,7 @@ package runtime
 import "unsafe"
 
 type mOS struct {
+	machport uint32 // return address for mach ipc
 	waitsema uint32 // semaphore for parking on locks
 }
 

src/runtime/runtime2.go

@@ -309,7 +309,6 @@ type m struct {
 	park          note
 	alllink       *m // on allm
 	schedlink     muintptr
-	machport      uint32 // return address for mach ipc (os x)
 	mcache        *mcache
 	lockedg       *g
 	createstack   [32]uintptr // stack that created this thread.