[AIX][CodeGen] Storage Locations for Constant Pointers

This patch adds an `llc` option `-mroptr` to specify storage locations for constant pointers on AIX.

When the `-mroptr` option is specified, constant pointers, virtual function tables, and virtual type tables are placed in read-only storage. Otherwise, by default, pointers, virtual function tables, and virtual type tables are placed are placed in read/write storage.

https://reviews.llvm.org/D144190 enables the `-mroptr` option for `clang`.

Reviewed By: hubert.reinterpretcast, stephenpeckham, myhsu, MaskRay, serge-sans-paille

Differential Revision: https://reviews.llvm.org/D144189