target-sh4: fix fpu disabled/illegal exception

Illegal instructions in a slot delay should generate a slot illegal
instruction exception instead of an illegal instruction exception.

The current PC should be saved before generating such an exception,
but should not be corrected if in a delay slot, given it's already
done in the exception handler do_interrupt().

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
