test: add s390x case to nosplit test

Fixes this test on s390x.

Change-Id: Ie5b70e8191169867765ec9248d827ca12c6405f4
Reviewed-on: https://go-review.googlesource.com/20964
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/test/nosplit.go b/test/nosplit.go
index 082fc3b..a59a3c3 100644
--- a/test/nosplit.go
+++ b/test/nosplit.go
@@ -275,6 +275,9 @@
 		case "amd64":
 			ptrSize = 8
 			fmt.Fprintf(&buf, "#define REGISTER AX\n")
+		case "s390x":
+			ptrSize = 8
+			fmt.Fprintf(&buf, "#define REGISTER R10\n")
 		default:
 			fmt.Fprintf(&buf, "#define REGISTER AX\n")
 		}