LLEXT: Xtensa: add a Kconfig option to enable -fPIC

Currently when building LLEXT for Xtensa we use the -fPIC compiler
option, but this cannot be used when using detached sections in
extensions. Add a Kconfig option to switch between the two
compilation modes and switch -fPIC off when building relocatable
(partially linked) ELF binaries.

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