[AArch64] Add function comments to some prologue/epilogue helpers

[AArch64] Add function comments to some prologue/epilogue helpers

	* config/aarch64/aarch64.c (aarch64_register_saved_on_entry): Add
	function comment.
	(aarch64_next_callee_save): Likewise.
	(aarch64_pushwb_single_reg): Likewise.
	(aarch64_gen_storewb_pair): Likewise.
	(aarch64_push_regs): Likewise.
	(aarch64_gen_loadwb_pair): Likewise.
	(aarch64_pop_regs): Likewise.
	(aarch64_gen_store_pair): Likewise.
	(aarch64_gen_load_pair): Likewise.
	(aarch64_save_callee_saves): Likewise.
	(aarch64_restore_callee_saves): Likewise.

From-SVN: r241777