llext: xtensa: add support for in-place relocatable extensions

Currently LLEXT on Xtensa supports relocatable extensions, linked for
a specific address range, while relocation itself takes place in a
temporary buffer. For this section addresses have to be set correctly
by the linker for their target locations.

This commit adds support for relocatable extensions, built without
using specific memory addresses and run at the same addresses, where
they are loaded.

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