gdb: more current_program_space->core_bfd() removal

gdb: more current_program_space->core_bfd() removal

This commit changes the signature of the gdbarch_core_info_proc method
so that it takes a 'struct bfd *' as an extra argument.  This argument
is used to pass through the core file bfd pointer.

Now, in corelow.c, when calling gdbarch_core_info_proc, we can pass
through current_program_space->core_bfd() as the argument.  Within the
implementations, (Linux and FreeBSD) we can use this argument rather
than having to access the core file through current_program_space.

This reduces the use of global state, which I think is a good thing.

There should be no user visible changes after this commit.

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