[llvm-ar] Fix for handling thin archive with SYM64 and a test case for it

WHen thin archives are created which have symbol table of type SYM64 then all the tools will not work since they cannot read the files properly.
One can reproduce the problem as follows:
1. Take a hello world program and create an archive out of it. The SYM64_THRESHOLD=0 will force the generation of SYM64 symbol table.
    clang -c hello.cpp
    SYM64_THRESHOLD=0 llvm-ar crsT mylib.a hello.o
2. Now try to use any of the tools on this mylib.a and it will fail.
    llvm-nm -M mylib.a

THis fix will eliminate these failures. A regression test is created in llvm/test/Object/archive-symtab.test

Reviewed By: MaskRay, Ramesh

Differential Revision: https://reviews.llvm.org/D107322