Fix definition of PAGESIZE and B_PAGE_SIZE

On sparc, the minimal page size we can use is 8K. Since B_PAGE_SIZE and
PAGESIZE defines were hardcoded to 4K, this resulted in a lot of
confusion in all code trying to manipulate pages.

- Remove cpu.h from headers/private/kernel/arch/*. It dates back from
NewOS and was not used anymore since our kernel uses B_PAGE_SIZE
(PAGE_SIZE was the only thing defined in this header).
- Add posix/arch/*/limits.h with the arch specific page size and include
it from the main limits.h.
- Adjust bios_ia32/debug.cpp which was the only place using the
PAGE_SIZE constant from the deleted headers.
- Change OS.h to define B_PAGE_SIZE to be the same as POSIX PAGESIZE.
- Define PAGESIZE in the build header if the host OS doesn't.

Change-Id: I8c3732cf952ea3c2f088aa16d216678fbf198b96
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3558
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>