target/sparc: Clear may_lookup for npc == DYNAMIC_PC

With pairs of jmp+rett, pc == DYNAMIC_PC_LOOKUP and
npc == DYNAMIC_PC.  Make sure that we exit for interrupts.

Cc: qemu-stable@nongnu.org
Fixes: 633c4283 ("target/sparc: Introduce DYNAMIC_PC_LOOKUP")
Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>