Introduce program_space::add_objfile

Introduce program_space::add_objfile

This introduces a new method, program_space::add_objfile, that adds an
objfile to the program space's list of objfiles.  It also changes the
obfile's constructor so that linking an objfile into this list is not
done here.

The former is an improvement because it makes more sense to treat the
program space as a container holding objfiles -- so manipulation of
the list belongs there.

The latter is not strictly needed, but seemed better both because it
is removing a global side effect from a constructor, and for symmetry
reasons, as a subsequent patch will remove unlinking from the
destructor.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* progspace.h (struct program_space) <add_objfile>: Declare
	method.
	* progspace.c (program_space::add_objfile): New method.
	* objfiles.c (~objfile): Don't unlink objfile.
	(put_objfile_before): Remove.
	(add_separate_debug_objfile): Don't call put_objfile_before.
	(objfile::make): Call add_objfile.  Set new_objfiles_available on
	the per-program-space data.

Change-Id: I93e8525dda631cb89dcc2046a5c51c7c9f34ccfd