llext: add wrapper to EXPORT_SYMBOL / LL_EXTENSION_SYMBOL

This patch adds an indirection level moving the current code from inside
the EXPORT_SYMBOL and LL_EXTENSION_SYMBOL macros to private Z_* macros.
This is done to allow for the official APIs to properly expand more
complex arguments, such as the DT function-like macros.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>