x/sys/unix: fix time/utime/utimes on arm64

In commit "7e44b69 x/sys/unix: fix invalid syscall on linux/arm"
a test was added for time/utime syscall. This test exposed that
neither time/utime work on arm64, because they call the legacy
syscall "utimes". As a new architecture, arm64 doesn't implement
any legacy syscalls.

Implement by first calling utimensat, using UtimesNano as exampple.

Change-Id: Iffed410730c06ac4c8184241d16eebf08c367524
Reviewed-on: https://go-review.googlesource.com/20174
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_arm64.go

@@ -70,9 +70,6 @@ func Lstat(path string, stat *Stat_t) (err error) {
 func Getpagesize() int { return 65536 }
 
 //sysnb	Gettimeofday(tv *Timeval) (err error)
-//sysnb	Time(t *Time_t) (tt Time_t, err error)
-
-//sys	Utime(path string, buf *Utimbuf) (err error)
 
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
 
@@ -91,6 +88,26 @@ func NsecToTimeval(nsec int64) (tv Timeval) {
 	return
 }
 
+func Time(t *Time_t) (Time_t, error) {
+	var tv Timeval
+	err := Gettimeofday(&tv)
+	if err != nil {
+		return 0, err
+	}
+	if t != nil {
+		*t = Time_t(tv.Sec)
+	}
+	return Time_t(tv.Sec), nil
+}
+
+func Utime(path string, buf *Utimbuf) error {
+	tv := []Timeval{
+		{Sec: buf.Actime},
+		{Sec: buf.Modtime},
+	}
+	return Utimes(path, tv)
+}
+
 func Pipe(p []int) (err error) {
 	if len(p) != 2 {
 		return EINVAL