runtime: use conventional shift style for gcBitsChunkBytes

The convention for writing something like "64 kB" is 64<<10, since
this is easier to read than 1<<16. Update gcBitsChunkBytes to follow
this convention.

Change-Id: I5b5a3f726dcf482051ba5b1814db247ff3b8bb2f
Reviewed-on: https://go-review.googlesource.com/23132Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mheap.go

@@ -1243,7 +1243,7 @@ func freespecial(s *special, p unsafe.Pointer, size uintptr) {
 	}
 }
 
-const gcBitsChunkBytes = uintptr(1 << 16)
+const gcBitsChunkBytes = uintptr(64 << 10)
 const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{})
 
 type gcBitsHeader struct {