[SystemZ][z/OS] Introduce initial support for GOFF asm parser

- Introduce a skeleton outline for the GOFFAsmParser
- Before instantiating AsmParser/HLASMAsmParser, target specific asm parsers are attempted to be initialized first before proceeding. If it doesn't exist for a particular file type, we report a fatal error.
- This patch allows to properly instantiate the HLASMAsmParser on z/OS, and ensures we can write lit tests and unit tests which will involve the instantiation of asm parsers, without an assert / fatal error.

Reviewed By: uweigand, Kai

Differential Revision: https://reviews.llvm.org/D110730