xinstall: Don't use UF_SETTABLE to infer whether st_flags is present

A future commit will add definitions of these for use in bootstrap tools
on Linux, but struct stat will still be the host's that lacks st_flags.
Switch this to just check for Linux instead, matching nbtool_config.h.

Reviewed by: emaste, markj, imp
Differential Revision: https://reviews.freebsd.org/D50079