unix: call gettimeofday libc function directly

This also works when invoking the raw syscall on Go ≤ 1.11

Fixes golang/go#38925

Change-Id: I137532da92b457a6853ed1be87c0aa72d2b2c886
Reviewed-on: https://go-review.googlesource.com/c/sys/+/232803
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_darwin_arm.go

@@ -20,17 +20,6 @@ func setTimeval(sec, usec int64) Timeval {
 	return Timeval{Sec: int32(sec), Usec: int32(usec)}
 }
 
-//sysnb	gettimeofday(tp *Timeval) (sec int32, usec int32, err error)
-func Gettimeofday(tv *Timeval) (err error) {
-	// The tv passed to gettimeofday must be non-nil
-	// but is otherwise unused. The answers come back
-	// in the two registers.
-	sec, usec, err := gettimeofday(tv)
-	tv.Sec = int32(sec)
-	tv.Usec = int32(usec)
-	return err
-}
-
 func SetKevent(k *Kevent_t, fd, mode, flags int) {
 	k.Ident = uint32(fd)
 	k.Filter = int16(mode)