[Object] Support reading 64-bit MIPS ELF archives

The 64-bit MIPS ELF archive file format is used by MIPS64 targets.
The main difference from a regular archive file is the symbol table format:
1. ar_name is equal to "/SYM64/"
2. number of symbols and offsets are 64-bit integers

http://techpubs.sgi.com/library/manuals/4000/007-4658-001/pdf/007-4658-001.pdf
Page 96

The patch allows reading of such archive files by llvm-nm, llvm-objdump
and other tools. But it does not support archive files with number of symbols
and/or offsets exceed 2^32. I think it is a rather rare case requires more
significant modification of `Archive` class code.

http://reviews.llvm.org/D7546

llvm-svn: 229520