modpost: error out if addend_*_rel() is not implemented for REL arch

The section mismatch check relies on the relocation entries.

For REL, the addend value is implicit, so we need some code to compute
it. Currently, EM_386, EM_ARM, and EM_MIPS are supported. This commit
makes sure we covered all the cases.

I believe the other architectures use RELA, where the explicit r_addend
field exists.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>