unix: add Select syscall on AIX

As select is a keyword of Go, C.select cannot be used on gccgo files.
Therefore, select call is transformed into c_select.

Change-Id: Ic68a7ffa03dc5a5d70514876b5f6afb565691a4a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/172739
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_aix_test.go

@@ -123,6 +123,26 @@ func TestUtimesNanoAt(t *testing.T) {
 	}
 }
 
+func TestSelect(t *testing.T) {
+	_, err := unix.Select(0, nil, nil, nil, &unix.Timeval{Sec: 0, Usec: 0})
+	if err != nil {
+		t.Fatalf("Select: %v", err)
+	}
+
+	dur := 150 * time.Millisecond
+	tv := unix.NsecToTimeval(int64(dur))
+	start := time.Now()
+	_, err = unix.Select(0, nil, nil, nil, &tv)
+	took := time.Since(start)
+	if err != nil {
+		t.Fatalf("Select: %v", err)
+	}
+
+	if took < dur {
+		t.Errorf("Select: timeout should have been at least %v, got %v", dur, took)
+	}
+}
+
 func TestPselect(t *testing.T) {
 	if runtime.GOARCH == "ppc64" {
 		t.Skip("pselect issue with structure timespec on AIX 7.2 tl0, skipping test")