os/signal: disable loading of history during test

Bring in https://golang.org/cl/98616 from gc tip.
    
    Original CL description:
    
        This change modifies Go to disable loading of users' shell history for
        TestTerminalSignal tests. TestTerminalSignal, as part of its workload,
        will execute a new interactive bash shell. Bash will attempt to load the
        user's history from the file pointed to by the HISTFILE environment
        variable. For users with large histories that may take up to several
        seconds, pushing the whole test past the 5 second timeout and causing
        it to fail.
    
    Reviewed-on: https://go-review.googlesource.com/107624

From-SVN: r259452

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-2c7093358e5f5ebeb102d44d1036ca0a807d46a5
+b367349d85f315e94e10ee2d76a7c6a46b993dcb
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/os/signal/signal_cgo_test.go

@@ -89,6 +89,8 @@ func TestTerminalSignal(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
 	cmd := exec.CommandContext(ctx, bash, "--norc", "--noprofile", "-i")
+	// Clear HISTFILE so that we don't read or clobber the user's bash history.
+	cmd.Env = append(os.Environ(), "HISTFILE=")
 	cmd.Stdin = slave
 	cmd.Stdout = slave
 	cmd.Stderr = slave