Add the core architecture for the lld driver.

This includes selecting which driver to emulate, option parsing, invocation
building, and running the link. This currently only supports a very basic
subset of ld for x86_64-linux.

lld -flavor ld obj.o -o obj

or symlink lld as (ld , link, ld64, core) to get the desired behavior without
-flavor.

llvm-svn: 169659