libelf: correct mips64el test to use ELF header

libelf maintains two views of endianness: e_byteorder, and
e_ident[EI_DATA] in the ELF header itself. e_byteorder is not always
kept in sync, so use the ELF header endianness to test for mips64el.

PR: 231790
Bisected by: sbruno
Reviewed by: jhb
Approved by: re (kib)
MFC with: r338478
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17380