llext: set proper LLEXT_STORAGE default for Xtensa

This patch sets the default value for LLEXT_STORAGE_WRITABLE to 'y' on
the Xtensa architecture. This is necessary because it does not currently
support the read-only mode for the LLEXT storage.

Make sure the default reflects this instead of asking the user to
manually set it.

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