libelf: correct mips64 test to use ELF header

[emaste]

Summary

libelf maintains two views of endianness: e_byteorder, and e_ident[EI_DATA] in the ELF header itself. e_byteorder is not necessarily always in sync, so use the ELF header endianness to test for mips64le.

PR: 231790
See also D15734

[jhb]

Use "mips64el" instead of "mips64le" in the commit message. Good sleuthing!

[jkoshy_users.sourceforge.net]

We would also need to check for 64bit-ness prior to using the e_hdr64 field.

[emaste]

In D17380#370866, @jkoshy_users.sourceforge.net wrote:

We would also need to check for 64bit-ness prior to using the e_hdr64 field.

Note that the original change in D15734 only calls _libelf_is_mips64el in 64-bit cases so in practice it won't be an issue (the test already checked e_ehdr64->e_machine), and I hope we can soon replace it with a general translator as you mentioned earlier. But I could add a safety-belt until that happens.