cmd/internal/gc, cmd/7g: use zero register in Componentgen

Fixes #9855

Use an architectural zero register as the source for zeroing, if available.

Change-Id: Ie5b4ba4e3d356c6f892bfd1cebd14d5152bdeeb0
Reviewed-on: https://go-review.googlesource.com/8722
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/src/cmd/internal/gc/go.go b/src/cmd/internal/gc/go.go
index 5803f39..25ada55 100644
--- a/src/cmd/internal/gc/go.go
+++ b/src/cmd/internal/gc/go.go
@@ -769,6 +769,7 @@
 	REGRETURN    int // AX
 	REGMIN       int
 	REGMAX       int
+	REGZERO      int // architectural zero register, if available
 	FREGMIN      int
 	FREGMAX      int
 	MAXWIDTH     int64