runtime: fix nacl build, hashmap overflow field offset was incorrect.

Change-Id: Ieb305b2a4d4ef28d70a8b8ece703f495c5af0529
Reviewed-on: https://go-review.googlesource.com/2051Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/hashmap.go

@@ -150,10 +150,10 @@ func evacuated(b *bmap) bool {
 }
 
 func (b *bmap) overflow(t *maptype) *bmap {
-	return *(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize) - ptrSize))
+	return *(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize) - regSize))
 }
 func (b *bmap) setoverflow(t *maptype, ovf *bmap) {
-	*(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize) - ptrSize)) = ovf
+	*(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize) - regSize)) = ovf
 }
 
 func makemap(t *maptype, hint int64) *hmap {