m32r: Define symbols to unify platform-dependent ICU checks

On some m32r platforms, cascaded ICUs are used.
This patch is required to simplify ei_handler and consolidate platform-
dependent ICU check routines.

  platform       ICU/INT1  ICU/INT0  ICU/INT2
 --------------  --------  --------  --------
  m32104ut        o         -         -
  m32700ut        o         o         o
  opsput          o         o         o
  usrv            o         -         -
  (others)        -         -         -

Signed-off-by: Hitoshi Yamamoto <hitoshiy@linux-m32r.org>
Signed-off-by: Hirokazu Takata <takata@linux-m32r.org>