gm2-lang.cc tidyup post review.

gm2-lang.cc tidyup post review.

Removed misleading comments, use a simplified
gm2_eh_peronality_decl borrowed from cp.  Addressed the comments
made during the code review:
https://gcc.gnu.org/pipermail/gcc-patches/2022-November/606694.html.

gcc/ChangeLog:

	* m2/gm2-lang.cc: Removed misleading comments.
	(m2_write_global_declarations) Rewritten.
	(gm2_langhook_type_for_mod) Rewritten the case statements
	into more portable code.  (personality_decl) Removed.
	(gm2_eh_personality_decl) New tree decl.
	(gm2_langhook_eh_personality) Rewritten.
	(gm2_langhook_init) Do not create void_list_node as it is
	created by build_common_builtin_nodes.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>