LLD: Introduce a GNU LD style driver for COFF

When building COFF programs many targets such as mingw prefer
to have a gnu ld frontend. Rather then having a fully fledged
standalone driver we wrap a shim around the LINK driver.

Extra tests were provided by mstorsjo

Reviewers: mstorsjo, ruiu

Differential Revision: https://reviews.llvm.org/D33880

llvm-svn: 312926