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.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@169659 91177308-0d34-0410-b5e6-96231b3b80d8