Fix arm-netbsd build error: convert from FPA to VFP

Fix arm-netbsd build error: convert from FPA to VFP

The floating point register interface has changed to this:
https://github.com/NetBSD/src/blob/trunk/sys/arch/arm/include/reg.h

It now uses VFP instead of FPA registers. This patch updates
arm-nbsd-nat.c accordingly.

Also implements read_description so that these registers are correctly
printed by "info registers" et al.

Tested by compiling & running on arm-netbsd on qemu.

gdb/ChangeLog:

2020-03-02  Christian Biesinger  <cbiesinger@google.com>

	* arm-nbsd-nat.c (arm_supply_fparegset): Rename to...
	(arm_supply_vfpregset): ...this, and update to use VFP registers.
	(fetch_fp_register): Update.
	(fetch_fp_regs): Update.
	(store_fp_register): Update.
	(store_fp_regs): Update.
	(arm_netbsd_nat_target::read_description): New function.
	(fetch_elfcore_registers): Update.