system/boot: More cleanup and shuffling

* arm efi additions
* cleanup some cpu headers which were oddly
split between efi and bios_ia32
* Move calculate_cpu_conversion_factor over to
arch_timer since it is timerish, and x86 only
* Drop some duplicated code from efi start. Move
hpet init code into efi timer/hpet code

Change-Id: Ia4264a5690ba8c09417b06788febc4f572f111ce
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2259
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

EFI: add cpu.cpp from bios_ia32