Follow golang.org/cl/55130 and add the utimensat syscall on *BSD. Use it
in UtimesNano and UtimesNanoAt (which is added on *BSD, akin to the
already existing implementation on Linux and Solaris).

Also add AT_FDCWD and AT_SYMLINK_NOFOLLOW where they are missing. These
might be used with UtimesNanoAt.

In order to be able to generate the syscalls, also add two missing
$GOOS_$GOARCH patterns to mkall.sh. As a side effect, some additional
syscalls are added for openbsd/arm.

Change-Id: I85351098002209f8454ec328cef0cfe9d12c5214
Reviewed-on: https://go-review.googlesource.com/55071Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>