Add multi-arch capable 'fbsd_make_corefile_notes' variant

Add multi-arch capable 'fbsd_make_corefile_notes' variant

This creates a new version of the FreeBSD core file note generation
logic in the new target-dependent file "fbsd-tdep.c".  The new version
is mostly copied from "fbsd-nat.c", but uses the iterator instead of
regset_from_core_section and defines fbsd_make_corefile_notes as a
gdbarch method instead of a target method.

Consecutive architecture-dependent changes exploit the new version,
migrating away from the target method.  When all FreeBSD targets are
changed, the target method can go away.

gdb/ChangeLog:

	* fbsd-tdep.c: New file.
	* fbsd-tdep.h: New file.
	* Makefile.in (ALL_TARGET_OBS): Add fbsd-tdep.o.
	(HFILES_NO_SRCDIR): Add fbsd-tdep.h.
	(ALLDEPFILES): Add fbsd-tdep.c.