llext: add support for exporting symbols from extensions

Extensions should be able to selectively export their global symbols.
Add a LL_EXTENSION_SYMBOL() macro for that. Change the present
.sym_tab to be a temporary symbol table of all global symbols in an
extensions, used only during linking for internal purposes. Add a new
.exp_tab symbol table to store symbols, exported by an extension
permanently.

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