llvm: add llvm-dlltool support to the archiver

A PE COFF spec compliant import library generator.
Intended to be used with mingw-w64.

Supports:
PE COFF spec (section 8, Import Library Format)
PE COFF spec (Aux Format 3: Weak Externals)

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D29892

llvm-svn: 308329