Use core regset iterators on GNU Hurd

Use core regset iterators on GNU Hurd

Remove native-only core file handling on GNU Hurd.  Instead, enable the
x86 target generic core regset logic on the Hurd by providing an
appropriate register offset map.

Thanks to Samuel Thibault for testing!

gdb/

	* config/i386/i386gnu.mh (NATDEPFILES): Remove core-regset.o.
	* i386gnu-nat.c: Do not include <sys/procfs.h> or "gregset.h".
	(CREG_OFFSET, creg_offset, CREG_ADDR): Remove.
	(supply_gregset, supply_fpregset): Remove.
	* i386gnu-tdep.c (i386gnu_gregset_reg_offset): New variable.
	(i386gnu_init_abi): Set tdep->gregset_reg_offset, gregset_num_regs,
	and sizeof_gregset.