Fix build for aarch64, arm, and riscv FreeBSD native targets.

Fix build for aarch64, arm, and riscv FreeBSD native targets.

Remove unused gdbarch argument to helper functions originally
copied from mips-fbsd-nat.c.  Include regcache.h previously
included from defs.h via gdbarch.h.

gdb/ChangeLog:

	* aarch64-fbsd-nat.c: Include regcache.h.
	(getregs_supplies, getfpregs_supplies): Remove unused gdbarch
	argument.
	(aarch64_fbsd_nat_target::fetch_registers)
	(aarch64_fbsd_nat_target::store_registers): Remove gdbarch
	variable.
	* arm-fbsd-nat.c, riscv-fbsd-nat.c: Likewise.