llvm-ar: Darwin archive format fixes.

* Support writing the DARWIN64 symbol table format.

* In darwin archives, emit a symbol table whenever requested, even
  when there are no members, as the apple linker will abort if given
  an archive without a symbol table.

Added tests for same, and also simplified and moved the GNU 64-bit
symbol table test into archive-symtab.test.

llvm-svn: 344183