Move core_bfd to program space

Move core_bfd to program space

This moves the core_bfd global to be a field of the program space.  It
then replaces core_bfd with a macro to avoid a massive patch -- the
same approach taken for various other program space fields.

This is a basic transformation for multi-target work.

2018-05-11  Tom Tromey  <tromey@redhat.com>
	    Pedro Alves  <tromey@redhat.com>

	* corefile.c (core_bfd): Remove.
	* gdbcore.h (core_bfd): Now a macro.
	* progspace.h (struct program_space) <cbfd>: New field.