gdb: pass core file through gdbarch API when loading shared libraries

Continuing the removal of 'current_program_space->core_bfd ()' from
GDB, this commit updates two gdbarch methods:

  gdbarch_core_xfer_shared_libraries
  gdbarch_core_xfer_shared_libraries_aix

to take the core file BFD as a reference parameter.  For now this just
moves the 'current_program_space->core_bfd ()' calls up the program
stack into core_target::xfer_partial.  In the future I plan to move
the core file BFD object out of the program_space and into the
core_target, at which point these new global accesses can also be
removed.

There should be no user visible changes after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>