llext: use elf_rela_t instead of elf_rel_t

elf_rela_t contains elf_rel_t exactly and contains an additional
field at the end. Therefore pointers of that type can be used for
both types, making the code generic.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>