Change arm_objfile_data_key to use type-safe registry

Change arm_objfile_data_key to use type-safe registry

After seeing Simon's patch to change arm_per_objfile to use new and
delete, I realized it is now simple to change arm_objfile_data_key to
use the type-safe registry.

gdb/ChangeLog
2019-06-27  Tom Tromey  <tromey@adacore.com>

	* arm-tdep.c (arm_objfile_data_key): Move lower.  Change type to
	objfile_key.
	(arm_find_mapping_symbol, arm_record_special_symbol)
	(_initialize_arm_tdep): Update.
	(arm_objfile_data_free): Remove.