Add initial piece for softraid(4) support on arm64

Add initial piece for softraid(4) support on arm64

arm64 is the only currently supported OpenBSD platform which both
a) supports booting off root on softraid(4) (kernel and bootloader) and
b) is an EFI platform (as far as installboot(8) is concerned).

Currently, installboot treats softraid root volumes as regular devices,
ignoring ignores chunk devices completely.

Teach installboot the first bits of softraid support for EFI:
installing the single-stage boot loader on chunks rather than the volume.

Copy over sparc64's softraid stage-1 code as-is and make its stage-2 a NOOP:

	# ./obj/installboot -v sd4
	Using / as root
	installing bootstrap on /dev/rsd4c
	using first-stage /usr/mdec/BOOTAA64.EFI
	sd4: softraid volume with 1 disk(s)
	sd0a: installing boot blocks on /dev/rsd0c
	copying /usr/mdec/BOOTAA64.EFI to /tmp/installboot.KuBD4zkfpM/efi/boot/bootaa64.efi
	writing /tmp/installboot.KuBD4zkfpM/efi/boot/startup.nsh

arm64 miniroot fits and boots with this.

OK stsp

As of now, EFI partitions must still be created manually as installboot's
'-p' does not support softraid at all (next missing piece for root on
softraid on arm64 installations to work out-of-the-box).