Change arm-tdep.c to use type-safe registry

Change arm-tdep.c to use type-safe registry

This changes arm-tdep.c to use the type-safe registry, removing a use
of VEC in the process.

2019-07-10  Tom Tromey  <tromey@adacore.com>

	* arm-tdep.c (arm_exidx_entry_s): Remove typedef.  Don't define
	VEC.
	(struct arm_exidx_entry): New method operator<.
	(struct arm_exidx_data) <section_maps>: Change type.
	(arm_exidx_data_free): Remove.
	(arm_exidx_data_key): Change type.  Move lower.
	(arm_exidx_new_objfile): Update.
	(arm_compare_exidx_entries): Remove.
	(arm_find_exidx_entry, _initialize_arm_tdep)