re PR go/86331 (the gccgo's "go" tool looks like failing to invoke any sub go command)

	PR go/86331
    os: check return value as well as error from waitid
    
    https://gcc.gnu.org/PR86331 indicates that if a signal handler runs it
    is possible for syscall.Syscall6 to return a non-zero errno value even
    if no error occurs. That is a problem in general, but this fix will
    let us work around the general problem for the specific case of
    calling waitid.
    
    Reviewed-on: https://go-review.googlesource.com/121595

From-SVN: r262313

gcc/go/gofrontend/MERGE

-e1fcce0aec27b1f50ac0e736f39f4c806c2a5baa
+94738979a3422e845acf358a766aabf8b9275d43
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/os/wait_waitid.go

@@ -28,9 +28,12 @@ func (p *Process) blockUntilWaitable() (bool, error) {
 	// We don't care about the values it returns.
 	var siginfo [16]uint64
 	psig := &siginfo[0]
-	_, _, e := syscall.Syscall6(syscall.SYS_WAITID, _P_PID, uintptr(p.Pid), uintptr(unsafe.Pointer(psig)), syscall.WEXITED|syscall.WNOWAIT, 0, 0)
+	r, _, e := syscall.Syscall6(syscall.SYS_WAITID, _P_PID, uintptr(p.Pid), uintptr(unsafe.Pointer(psig)), syscall.WEXITED|syscall.WNOWAIT, 0, 0)
 	runtime.KeepAlive(p)
-	if e != 0 {
+	// Check r as well as e because syscall.Syscall6 currently
+	// just returns errno, and the SIGCHLD signal handler may
+	// change errno. See https://gcc.gnu.org/PR86331.
+	if r != 0 && e != 0 {
 		// waitid has been available since Linux 2.6.9, but
 		// reportedly is not available in Ubuntu on Windows.
 		// See issue 16610.