In commit "7e44b69d 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/20174Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>