cmd/6g: fix build for nacl/amd64p32

Change-Id: I3de78f321c99f8f850a304f7d34c85d0fe254c0e
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7975
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/6g/galign.go

@@ -65,7 +65,7 @@ func betypeinit() {
 
 func main() {
 	if obj.Getgoos() == "nacl" {
-		resvd = append(resvd, x86.REG_BP, x86.REG_SI)
+		resvd = append(resvd, x86.REG_BP, x86.REG_R15)
 	} else if obj.Framepointer_enabled != 0 {
 		resvd = append(resvd, x86.REG_BP)
 	}