arm: Avoid UB in elf_machine_rel()

This recently came up during a cleanup to remove misaligned accesses
from the RISC-V port.

Link: https://sourceware.org/pipermail/libc-alpha/2022-June/139961.html


Suggested-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Fangrui Song <maskray@google.com>