[AArch64] insert_mov_immed_arch does not store encoded address of src_inst

insert_mov_immed_arch is supposed to store the encoded address of src_inst, when the arg is not NULL. But it uses encode_estimate instead.

The ARM32 implementation does the right thing.

This issue also manifests when using instrlist_insert_mov_instr_addr which relies on insert_mov_immed_arch.