[AsmParser][SystemZ][z/OS] Use updated framework in AsmLexer to accept special...

[AsmParser][SystemZ][z/OS] Use updated framework in AsmLexer to accept special tokens as Identifiers

- Previously, https://reviews.llvm.org/D99889 changed the framework in the AsmLexer to treat special tokens, if they occur at the start of the string, as Identifiers.
- These are used by the MASM Parser implementation in LLVM, and we can extend some of the changes made in the previous patch to SystemZ.
- In SystemZ, the special "tokens" referred to here are "_", "$", "@", "#". [_|$|@|#] are already supported as "part" of an Identifier.
- The changes in this patch ensure that these special tokens, when they occur at the start of the Identifier, are treated as Identifiers.

Reviewed By: abhina.sreeskantharajan

Differential Revision: https://reviews.llvm.org/D100959