PPC nativeGen: Add support for MO_SS_Conv_W32_W64
PPC nativeGen: Add support for MO_SS_Conv_W32_W64

This is required by D4363. D4362 has the implementation for i386
this commit adds PowerPC.

Test Plan: validate

Reviewers: erikd, hvr, simonmar, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4468

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc