rtld: fix check for endianess of elf hints file

Don't check if the elf hints file is in host byte order, but check
if it is in little endian by looking at the magic number.
This fixes rtld on big endian platforms.
Reviewed by: se, kib (prior version of the patch)
Fixes: 7b77d37a561b ("rtld-elf: support either byte-order of hints")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D44472