unix: add utimensat and use it for UtimesNano on BSD

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/55071
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_openbsd.go

@@ -193,6 +193,7 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 //sys	munmap(addr uintptr, length uintptr) (err error)
 //sys	readlen(fd int, buf *byte, nbuf int) (n int, err error) = SYS_READ
 //sys	writelen(fd int, buf *byte, nbuf int) (n int, err error) = SYS_WRITE
+//sys	utimensat(dirfd int, path string, times *[2]Timespec, flags int) (err error)
 
 /*
  * Unimplemented
@@ -282,6 +283,5 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 // thrsleep
 // thrwakeup
 // unlinkat
-// utimensat
 // vfork
 // writev