Boilerplate for producing XCOFF object files from the PowerPC backend.

Boilerplate for producing XCOFF object files from the PowerPC backend.

Stubs out a number of the classes needed to produce a new object file format
(XCOFF) for the powerpc-aix target. For testing input is an empty module which
produces an object file with just a file header.

Differential Revision: https://reviews.llvm.org/D61694

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365541 91177308-0d34-0410-b5e6-96231b3b80d8