syscall: change clone argument order on s390x

The Linux ABI takes arguments in a different order on s390x.

Change-Id: Ic9cfcc22a5ea3d8ef77d4dd0b915fc266ff3e5f7
Reviewed-on: https://go-review.googlesource.com/20960Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/exec_linux.go

@@ -7,6 +7,7 @@
 package syscall
 
 import (
+	"runtime"
 	"unsafe"
 )
 
@@ -93,7 +94,11 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 	// About to call fork.
 	// No more allocation or calls of non-assembly functions.
 	runtime_BeforeFork()
+	if runtime.GOARCH == "s390x" {
+		r1, _, err1 = RawSyscall6(SYS_CLONE, 0, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0)
+	} else {
 		r1, _, err1 = RawSyscall6(SYS_CLONE, uintptr(SIGCHLD)|sys.Cloneflags, 0, 0, 0, 0, 0)
+	}
 	if err1 != 0 {
 		runtime_AfterFork()
 		return 0, err1