Allow xinstall and makefs to be crossbuilt on Linux and Mac

I need these tools in order to install the crossbuilt FreeBSD and create a
disk image. Linux does not have a st_flags in struct stat so unfortunately
I need a bunch of ugly ifdefs. The resulting binaries allow me to
sucessfully install a MIPS64 world and create a disk-image that boots.

Reviewed By: brooks, bdrewery, emaste
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D13307