llext: arch_elf_relocate: pass opval unmodified

The opval argument of arch_elf_relocate() was modified by
adding the value stored at opaddr before passing it to
arch_elf_relocate(). This presumed that the addend would
always be stored as a raw value at opaddr, which is not the
case for all relocation types.

This PR modifies opval to be the absolute address of opval,
and moves the addition of the addend from llext_link_plt()
to the implementation of arch_elf_relocate().

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>