gdb: pass core file to gdbarch_core_xfer_siginfo

gdb: pass core file to gdbarch_core_xfer_siginfo

Another patch that aims to remove 'current_program_space->core_bfd ()'
from GDB.  This time I'm passing the core file BFD as an argument to
the gdbarch method gdbarch_core_xfer_siginfo.

In corelow.c the core file is being passed, this does introduce a new
instance of 'current_program_space->core_bfd ()', but this is OK.  My
long term plan is to move the core bfd into core_target, in which case
the call to gdbarch_core_xfer_siginfo will have access to the core bfd
as a member variable.

For now though, this patch moves the accesses via global state up the
call stack, and consolidates the two calls from fbsd-tdep.c and
linux-tdep.c into the one call in corelow.c.

There should be no user visible changes after this commit.

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