boot: allocate cleanup pages below 4GiB only on x86

boot: allocate cleanup pages below 4GiB only on x86

Outside of x86, some machines (e.g. Apple silicon, AMD Opteron A1100) have
physical memory mapped above 4GiB, meaning this allocation will fail, causing
the entire boot process to fail on these machines.

This commit makes it so that the below-4GB address space allocation requirement
is only set on x86 platforms, and not on other platforms (that don't have the
specific Linux x86 boot protocol), thereby fixing boot on those that have no
memory mapped below 4GiB in their address space.

Tested on an Apple silicon M1 laptop and an AMD x86_64 desktop tower.

Fixes: #35026

Manual backport of 6e207b370e91e681efb08c497a6c8ad78e3c8d83.