llext: xtensa: strip the .xtensa.info section

.xtensa.info section isn't large so it was ignored in the beginning,
but its contents can differ depending on the build platform even if
the sources and the toolchain are the same. Strip the section to
restore binary reproducibility.

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