test: add s390x case to nosplit test

Fixes this test on s390x.

Change-Id: Ie5b70e8191169867765ec9248d827ca12c6405f4
Reviewed-on: https://go-review.googlesource.com/20964Reviewed-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>

test/nosplit.go

@@ -275,6 +275,9 @@ TestCases:
 		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")
 		}