boot/stub: allocate pages for combined initrds below 4GiB only on x86 (#35149)

boot/stub: allocate pages for combined initrds below 4GiB only on x86 (#35149)

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