scripts/cfify: refactor — hoist per-arch constants and helpers

Pure cleanup of scripts/cfify, no behavior change:

- Add a top-level module docstring describing what the script does.
- Fix the parse_reg docstring, which previously claimed to return a
  (prefix, number) tuple but actually returns a canonical register name.
- Hoist constants, compiled regex patterns, and helper functions out of
  the per-line while loop in add_cfi_directives() to module scope. The
  loop previously rebuilt them on every line of input. Applies to all
  three architectures: aarch64, x86_64, and the newly-added armv81m
  branch. Names are prefixed by architecture (AARCH64_*, X86_64_*,
  ARMV81M_*, armv81m_*) to keep the module namespace legible.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker