LLEXT: Xtensa: add support for L32R relocation

When building LLEXT for Xtensa with custom sections the compiler
can leave unresolved references in them. Then this has to be done by
the LLEXT core during linking. This commit adds linking support for
the L32R Xtensa instruction.

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