libelf: also test for 64-bit ELF in _libelf_is_mips64el

Although _libelf_is_mips64el is only called in contexts where we've
already checked that e_class is ELFCLASS64 but this may change in the
future. Add a safety belt so that we don't access an invalid e_ehdr64
union member if it does.

Reported by: jkoshy (in review D17380)