[LTO][AIX] Invoking AIX System Assembler in LTO CodeGen

This patch teaches LTOCodeGenerator to call into the AIX system assembler to generate object files. This is in contrast to the approach taken on other platforms, where the LTOCodeGenerate calls the integrated assembler to generate object files.  We need to rely on the system assembler because the integrated assembler is incomplete at the moment.

Reviewed By: w2yehia, MaskRay

Differential Revision: https://reviews.llvm.org/D134375