Stubs out TLOF for AIX and add support for common vars in assembly output.

Stubs out TLOF for AIX and add support for common vars in assembly output.

Stubs out a TargetLoweringObjectFileXCOFF class, implementing only
SelectSectionForGlobal for common symbols. Also adds an override of
EmitGlobalVariable in PPCAIXAsmPrinter which adds a number of defensive errors
and adds support for emitting common globals.

llvm-svn: 366727