Remove use of deprecated core functions (in NetBSD/ARM)

Remove use of deprecated core functions (in NetBSD/ARM)

This is in preparation for deleting deprecated_add_core_fns and
related code.

As a side-effect, this makes it possible to read NetBSD/ARM
core files on non-NetBSD/ARM platforms, subject to PR corefiles/25638.

I have removed this comment:
-  /* This is ok: we're running native...  */
Since we are using the gdbarch from the regcache, we should be
guaranteed to be calling the right function here, so it shouldn't
matter whether we are running native.

Tested by reading a NetBSD/ARM core file on Linux/x86-64 and NetBSD/ARM;
the "info registers" output matches the one from the system GDB.

gdb/ChangeLog:

2020-03-12  Christian Biesinger  <cbiesinger@google.com>

	* Makefile.in (HFILES_NO_SRCDIR): Add new arm-nbsd-tdep.h file.
	* arm-nbsd-nat.c (arm_supply_gregset): Moved to arm-nbsd-tdep and
	renamed to arm_nbsd_supply_gregset.
	(fetch_register): Update to call arm_nbsd_supply_gregset.
	(fetch_regs): Remove in favor of fetch_register with a -1 regno.
	(arm_netbsd_nat_target::fetch_registers): Update.
	(fetch_elfcore_registers): Removed.
	(_initialize_arm_netbsd_nat): Removed call to deprecated_add_core_fns.
	* arm-nbsd-tdep.c (struct arm_nbsd_reg): New struct.
	(arm_nbsd_supply_gregset): Moved from arm-nbsd-nat.c and updated to
	not require NetBSD system headers.
	(arm_nbsd_regset): New struct.
	(arm_nbsd_iterate_over_regset_sections): New function.
	(arm_netbsd_init_abi_common): Updated to call
	set_gdbarch_iterate_over_regset_sections.
	* arm-nbsd-tdep.h: New file.