target/sh4: ignore interrupts in a delay slot

Delay slots are indivisible, therefore avoid scheduling an interrupt in
the delay slot. However exceptions are possible.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>