riscv: Add support for HI20 PE relocations

The PE standard allows for HI20/LOW12 relocations. Within the efi_loader
target we always know that our relocation target is 4k aligned, so we
don't need to worry about the LOW12 part.

This patch adds support for the respective relocations. With this and a
few grub patches I have cooking in parallel I'm able to run grub on RISC-V.

Signed-off-by: Alexander Graf <agraf@suse.de>