llext: rework LL_EXTENSION_SYMBOL to generate const structs

The LL_EXTENSION_SYMBOL macro is used to export symbols defined in
extensions to the base image. This patch reworks the macro to use
`static const struct llext_const_symbol`, since:
- the associated values are semantically not modifiable;
- this exported symbol table is parsed by llext like section iterables,
  ignoring associated symbol information.

This is mostly a cosmetic change because, unlike the base image, these
symbols cannot be placed (and left) in ROM as they will most certainly
require runtime relocation. However, it makes this macro more consistent
with EXPORT_SYMBOL, which is the base image equivalent.

Also clarify some comments in the same header file.

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