stand/elf: Only support swapping headers on powerpc.

Powerpc is currently the only architecture that we support more than one
endian. It's the only one that benefits from this swapping, so restrict
the code to there. This saves about 1k in the i386 BIOS loader.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36577

(cherry picked from commit c0ecae78abbe5c925cd05d5e218aa6f038cd03c7)