gdb: remove core_bfd macro

The core_bfd macro hides a use of current_program_space.  Remove it, so
that we have the opportunity to get the program space from the context,
if possible.  I guess that the macro was introduced at some point to
replace a global variable of the same name without changing all the
uses.

Change-Id: I971a65b29b5e5a5941f3cb7ea234547daa787268
Approved-By: Tom Tromey <tom@tromey.com>
18 files changed