synctools: generalize asmwriter.py's AArch64-only alias/mnemonic logic

[StilesCrisis]

Problem

suite/synctools/asmwriter.py (the script that converts raw llvm-tblgen -gen-asm-writer output into capstone's C style) has AArch64-only logic hardcoded into several places, dating to when the script was written for AArch64 in 2022 and never exercised against another target since. Running it against a non-AArch64 target's raw tblgen output produced by a reasonably modern LLVM breaks in several ways - found while regenerating PowerPC's asm-writer output for #3052.

Bugs fixed

• getRegisterName/getMnemonic's "const char *AArch64InstPrinter::" skip rule was a literal string match, so for any other target it also matched (and silently swallowed) that target's own real getRegisterName definition line - e.g. PPC's "const char *PPCInstPrinter::getRegisterName(...)" matches the same substring pattern. Result: a getRegisterName body with no function signature at all. Now excludes getRegisterName explicitly, and matches the target's own class name instead of a hardcoded AArch64InstPrinter.
• getMnemonic had no translation for newer LLVM's early-out guard (if (Bits == 0) return {nullptr, Bits};), which doesn't exist in the older LLVM output this script was originally written against.
• Newer LLVM wraps register handles in MCRegister, so getRegisterName's raw definition now includes a RegNo = Reg.id(); redeclaration line that conflicts with this script's hardcoded plain-unsigned-parameter signature.
• The injected printAliasInstr alias-matching block hardcoded AArch64InstPrinterValidateMCOperand and AArch64's SME tile-slice-indexing bracket syntax (za[w8, 0]) unconditionally. Validator name is now parameterized by target; SME bracket handling now only applies for AArch64/ARM64, with a plain set_mem_access()-only fallback for every other target (none of which have SME syntax).

Each fix was confirmed necessary by an actual compiler/parse error while iterating toward a from-scratch PPC rebuild, not speculative.

Scope note

This is a synctools script fix, not a PPC-specific change - it generalizes logic that only worked for AArch64 to work for any target regenerated against a comparably modern LLVM. It's independent of #3052, which uses an older (LLVM 7.0.1-vintage) toolchain that doesn't hit any of these code paths at all.

One related note for anyone regenerating a target against modern LLVM specifically: if that target has zero K_Custom AliasPatternConds (true for PPC, as of writing), the injected <Target>InstPrinterValidateMCOperand call in the alias-matching block is unreachable but still needs a stub of that name in the target's InstPrinter.c to link - this script doesn't generate that stub itself, since it's C source, not template output.