Fix file_size on FreeBSD

It was using the generic UNIX `fstat` implemented in Odin, which is more
than what is needed here.

This also avoids the issue of needing a proper
`absolute_path_from_handle` implementation for it to work without error.

core/os/os_freebsd.odin

@@ -365,7 +365,7 @@ seek :: proc(fd: Handle, offset: i64, whence: int) -> (i64, Errno) {
 }
 
 file_size :: proc(fd: Handle) -> (i64, Errno) {
-	s, err := fstat(fd)
+	s, err := _fstat(fd)
 	if err != ERROR_NONE {
 		return -1, err
 	}