cmake: Reuse llext flags for EDK

Some flags are common between in tree extensions and out of tree
supported by the EDK. Instead of duplicating those flags, the EDK reuses
the llext ones.

However, as the EDK has its own needs, two new lists,
`LLEXT_EDK_APPEND_FLAGS` and `LLEXT_EDK_REMOVE_FLAGS` are defined to
allow EDK to append or remove flags as needed.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>