gdb: use current_program_space->core_bfd() a little less

gdb: use current_program_space->core_bfd() a little less

The function linux_read_core_file_mappings is passed an argument CBFD,
which is the BFD for the core file.  In
core_target::build_file_mappings, where the function is called, we
pass current_program_space->core_bfd() as the argument.

However, in linux_read_core_file_mappings, in some places we use the
CBFD argument, and in other places we directly use
current_program_space->core_bfd().  This is confusing, and
unnecessary.  Lets not do that.

Standardise on just using CBFD.  This removes some references to
global state in favour of passing the global state in as an argument,
I think this is a good thing.

There should be no user visible changes after this commit.

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