core: switch to LLVM integrated assembler by default

Summary:
Still support utilizing GAS via LOCAL_NO_INTEGRATED_AS_SUPPORT

Depends on D176

Test Plan: requires a few other changes which are WIP still

Reviewers: randall.rushing, anders3408, jriordan001

Reviewed By: anders3408

Differential Revision: http://review.codefirex.com/D430