runtime: round channel size in allocation instead of using system-specific pad field

Followup to CL 128700043.

LGTM=bradfitz, dvyukov
R=dvyukov, bradfitz
CC=golang-codereviews
https://golang.org/cl/133850043

src/pkg/runtime/chan.go

@@ -11,11 +11,11 @@ import "unsafe"
 
 const (
 	maxAlign  = 8
-	hchanSize = unsafe.Sizeof(hchan{})
+	hchanSize = unsafe.Sizeof(hchan{}) + uintptr(-int(unsafe.Sizeof(hchan{}))&(maxAlign-1))
 	debugChan = false
 )
 
-// TODO: make hchan.buf an unsafe.Pointer, not a *uint8
+// TODO(khr): make hchan.buf an unsafe.Pointer, not a *uint8
 
 func makechan(t *chantype, size int64) *hchan {
 	elem := t.elem

src/pkg/runtime/chan.h

@@ -21,9 +21,6 @@ struct	Hchan
 	byte*	buf;
 	uint16	elemsize;
 	uint32	closed;
-#ifndef GOARCH_amd64
-	uint32	pad;			// ensures proper alignment of the buffer that follows Hchan in memory
-#endif
 	Type*	elemtype;		// element type
 	uintgo	sendx;			// send index
 	uintgo	recvx;			// receive index