[Clang, SystemZ] Add support for option -mno-pic-data-is-text-relative.

Add support for this GCC option which has the purpose of disallowing text
relative accesses of module local symbols with PIC. In effect, this changes
the code model to "medium".

Reviewed By: uweigand, efriedma, MaskRay

Differential Revision: https://reviews.llvm.org/D137044