gdb: pass core file to gdbarch_core_read_x86_xsave_layout

Continuing the removal of 'current_program_space->core_bfd ()' from
GDB, this commit updates the gdbarch method
'gdbarch_core_read_x86_xsave_layout' 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::fetch_x86_xsave_layout.  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>