[SystemZ] Introduce distinction between the jg/jl family of mnemonics for GNU as vs HLASM

- This patch adds in the distinction between jg[*] and jl[*] pc-relative
  mnemonics based on the variant/dialect.
- Under the hlasm variant, we use the jl[*] family of mnemonics and under
  the att (GNU as) variant, we use the jg[*] family of mnemonics.
- jgnop which was added in https://reviews.llvm.org/D92185, is now restricted
  to att variant. jlnop is introduced and restricted to hlasm variant.
- The br[*]l additional mnemonics are mapped to either jl[*]/jg[*] based on
  the variant.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D97581