llext: fix handling of non-standard sections in relocatable case

When building partially linked / relocatable objects no ELF segments
are created and it becomes more difficult to predict which sections
the compiler will build and use. In this case a .data.rel.local
section is created by the compiler and it is needed to link .rodata
strings in a twister test. We can handle arbitrary sections at run-
time if .peek() is supported. If it isn't we need to allocate and
copy the section. For now we simply error out in such cases. Fixing
that would represent a larger change and can be done incrementally.

This also fixes the relocation calculation to point to the correct
symbol address instead of the memory location, where it's currently
residing, because that can be a temporary buffer as is the case with
SOF.

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