[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229520 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Object/Archive.h

@@ -178,6 +178,7 @@ public:
 
   enum Kind {
     K_GNU,
+    K_MIPS64,
     K_BSD,
     K_COFF
   };