llext: add context save and restore

Some applications need to save LLEXT context, e.g. when suspending,
to later restore it quickly without a full relinking. Add 2 functions
for context saving and restoring. Since these functions are likely to
change in the future, put them in llext_experimental.c, which depends
on CONFIG_LLEXT_EXPERIMENTAL and is disabled by default.

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