Add a minimal implementation of ranlib.

This is just enough to get "llvm-ranlib foo.a" working and tested. Making
llvm-ranlib a symbolic link to llvm-ar doesn't work so well with llvm's option
parsing, but ar's option parsing is mostly custom anyway.

This patch also removes the -X32_64 option. Looks like it was just added in
r10297 as part of implementing the current command line parsing. I can add it
back (with a test) if someone really has AIX portability problems without it.