Allow the kernel to be loaded at any 1MiB address. This requirement is
because we use the 1MiB section maps as they only need a single pagetable.

To allow this we only use pc relative loads to ensure we only load from
physical addresses until we are running from a known virtual address.

As a side effect any data from before or 64MiB after the kernel needs to
be mapped in to be used. This should not be an issue for kernels loaded
with ubldr as it places this data just after the kernel. It will be a
problem when loading directly from anything using the Linux ABI that
places the ATAG data outside this range, for example U-Boot.