gdb/jit: link to jit_objfile_data directly from the objfile struct

gdb/jit: link to jit_objfile_data directly from the objfile struct

Remove the use of objfile_data to associate a jit_objfile_data with an
objfile.  Instead, directly link to a jit_objfile_data from an objfile
struct.  The goal is to eliminate unnecessary abstraction.

The free_objfile_data function naturally becomes the destructor of
jit_objfile_data.  However, free_objfile_data accesses the objfile to
which the data is attached, which the destructor of jit_objfile_data
doesn't have access to.  To work around this, add a backlink to the
owning objfile in jit_objfile_data.  This is however temporary, it goes
away in a subsequent patch.

gdb/ChangeLog:
2020-07-22  Simon Marchi  <simon.marchi@polymtl.ca>

	* jit.h: Forward-declare `struct minimal_symbol`.
	(struct jit_objfile_data): Migrate to here from jit.c; also add a
	constructor, destructor, and an objfile* field.
	* jit.c (jit_objfile_data): Remove.
	(struct jit_objfile_data): Migrate from here to jit.h.
	(jit_objfile_data::~jit_objfile_data): New destructor
	implementation with code moved from free_objfile_data.
	(free_objfile_data): Delete.
	(get_jit_objfile_data): Update to use the jit_data field of objfile.
	(jit_find_objf_with_entry_addr): Ditto.
	(jit_inferior_exit_hook): Ditto.
	(_initialize_jit): Remove the call to
	register_objfile_data_with_cleanup.
	* objfiles.h (struct objfile) <jit_data>: New field.