Add an initial implementation of archive symbol table generation.

The symbol table has forward references in the file. Instead of allocating
a temporary buffer or counting the size and then writing, this implementation
writes a dummy value first and patches it once the final value is known.

There is room for performance improvement. I will implement them as soon as I
get some other features (like a ranlib mode) in.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186934 91177308-0d34-0410-b5e6-96231b3b80d8