system/boot: Finish cleaning up remaining platform_get_boot_partition

* Fixes the riscv64 build
* Should fix next_m68k loader (untested)

Change-Id: Ic9b2d4305302d28a9ca0c71f8e1e502c763162d9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4199
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

m68k: add next_m68k boot platform

Since the boot ROM API structure is declared with a custom alignment, we
simulate it with padding bytes around, and use aligntest.prg to verify
using ARAnyM + TOS/MiNT as we know how to link simple PRG files.

It now prints something to the screen then panics when initializing the
heap.

For now one must insert the loader manually into an existing floppy
image:
dd if=generated-m68k/objects/haiku/m68k/release/system/boot/next_m68k/haiku_loader.next_m68k bs=$((0x8000)) seek=1 of=next_floppy.img conv=notrunc

Change-Id: I06d74e9d85a352aab68dedce545bbe5fe9e990d5
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2220
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>