llext: make EXPORT_SYMBOL() universal

Currently LLEXT has two ways to export symbols:
EXPORT_SYMBOL() to exort symbols from the main firmware to extensions
LL_EXTENSION_SYMBOL() to export symbols from extensions
but it is possible to write code, that needs to export symbols
regardless of whether it is built into the main image or an
extension. And in fact there's no real need in two distinct macros -
we can use one for both cases and recognise at build time which
direction exporting is happening. This patch extends EXPORT_SYMBOL()
to also work from extensions.

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