Use adrp in the arm64 efi loader

On startup the arm64 efi loaders need to know PC-relative addresses.
Previously we used the adr instruction to find this address, however this
instruction is limited to +/- 1MiB.

Switch to adrp to find the 4k page the address is within and an add to
set the bottom 12 bits. This lets us address +/- 4GiB which should be
large enough for now.

Reported by: imp
MFC after: 2 weeks
Sponsored by: Innovate UK