Change jit.c to use type-safe registry

Change jit.c to use type-safe registry

This changes jit.c to use the type-safe registry.  Only one of the
registry keys in jit.c is converted; the other is trickier and so I've
left it be for now.

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

	* jit.c (jit_program_space_key): Change type.  Move lower.
	(get_jit_program_space_data): Update.
	(jit_program_space_data_cleanup): Remove.
	(jit_breakpoint_deleted, free_objfile_data, _initialize_jit):
	Update.
	(struct jit_program_space_data): Add initializers.