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_bsd.go

@@ -561,13 +561,19 @@ func Utimes(path string, tv []Timeval) error {
 
 func UtimesNano(path string, ts []Timespec) error {
 	if ts == nil {
+		err := utimensat(AT_FDCWD, path, nil, 0)
+		if err != ENOSYS {
+			return err
+		}
 		return utimes(path, nil)
 	}
-	// TODO: The BSDs can do utimensat with SYS_UTIMENSAT but it
-	// isn't supported by darwin so this uses utimes instead
 	if len(ts) != 2 {
 		return EINVAL
 	}
+	err := utimensat(AT_FDCWD, path, (*[2]Timespec)(unsafe.Pointer(&ts[0])), 0)
+	if err != ENOSYS {
+		return err
+	}
 	// Not as efficient as it could be because Timespec and
 	// Timeval have different types in the different OSes
 	tv := [2]Timeval{
@@ -577,6 +583,16 @@ func UtimesNano(path string, ts []Timespec) error {
 	return utimes(path, (*[2]Timeval)(unsafe.Pointer(&tv[0])))
 }
 
+func UtimesNanoAt(dirfd int, path string, ts []Timespec, flags int) error {
+	if ts == nil {
+		return utimensat(dirfd, path, nil, flags)
+	}
+	if len(ts) != 2 {
+		return EINVAL
+	}
+	return utimensat(dirfd, path, (*[2]Timespec)(unsafe.Pointer(&ts[0])), flags)
+}
+
 //sys	futimes(fd int, timeval *[2]Timeval) (err error)
 
 func Futimes(fd int, tv []Timeval) error {