llext: xtensa: fix relocations with multiple SLOT0_OP

Support for R_XTENSA_SLOT0_OP was implemented with a relocation table
test case, containing only one entry at offset 0. For multiple
entries .r_addend has to be taken into account.

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