runtime: align mcentral by cache line size

This may improve perormance during concurrent access
to mheap.central array from multiple CPU cores.

Change-Id: I8f48dd2e72aa62e9c32de07ae60fe552d8642782
Reviewed-on: https://go-review.googlesource.com/41550Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mheap.go

@@ -121,7 +121,7 @@ type mheap struct {
 	// gets its own cache line.
 	central [_NumSizeClasses]struct {
 		mcentral mcentral
-		pad      [sys.CacheLineSize]byte
+		pad      [sys.CacheLineSize - unsafe.Sizeof(mcentral{})%sys.CacheLineSize]byte
 	}
 
 	spanalloc             fixalloc // allocator for span*