arm64/insn: add support for emitting ADR/ADRP instructions

Add support for emitting ADR and ADRP instructions so we can switch
over our PLT generation code in a subsequent patch.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>