llext: fix symbol exporting for ET_REL

When building relocatable extensions, exported symbol names might end
up in a separate section (in my case ".rodata.str1.1") for which
storage address don't match section address. To access those names
addresses have to be relalculated.

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