gdb: remove most global core file accesses from fbsd-tdep.c

gdb: remove most global core file accesses from fbsd-tdep.c

This commit removes many places in fbsd-tdep.c where we access the
current core file via current_program_space, and replaces these
accesses with a function argument that is passed in.

There are still two uses of 'current_program_space->core_bfd ()' in
the file, these will be addressed in future work (not in this series
though).

There should be no user visible changes after this commit.

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