[SystemZ][z/OS] Add GOFF Support to the DataLayout

- This patch adds in the GOFF mangling support to the LLVM data layout string. A corresponding additional line has been added into the data layout section in the language reference documentation.
- Furthermore, this patch also sets the right data layout string for the z/OS target in the SystemZ backend.

Reviewed By: uweigand, Kai, abhina.sreeskantharajan, MaskRay

Differential Revision: https://reviews.llvm.org/D109362