Locale fix for endian big (EB) machines.

We have locale files generated on EL machines (e.g. during cross-build
on amd64 host), but then we are using them on EB machines (e.g. MIPS64EB),
so proceed byte-swap if necessary.

All the libc tests passed successfully, including Russian collation.

Tested by: br@, Hongyan Xia <hx242@cam.ac.uk>
Sponsored by: DARPA, AFRL
Sponsored by: HEIF5
Differential Revision: https://reviews.freebsd.org/D8281