[AArch64] Define GCS operations as SYS and SYSL aliases
Move the remaining `GCS` instructions from dedicated opcodes to `SYSxt/SYSLxt`
aliases, keeping a tied `SYSL` pseudo for codegen where `GCS` preserves the`
input register when disabled at runtime.
Update `GCS` intrinsic selection, scheduling, disassembly aliases, and MC
coverage for the generic `SYS/SYSL` encodings.
