kernel/boot/efi: Continue breaking apart arch-specific code; hpet

* Move x86 hpet behind timer interface.
* Add a few if x86,x86_64 macros to start.cpp.

Change-Id: I583ec1b064785182e6d48dfbcd91b1bb2ead4b44
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1929
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>