linux-user: arm: Handle (ignore) EXCP_YIELD in ARM cpu_loop()

The new-in-ARMv8 YIELD instruction has been implemented to throw
an EXCP_YIELD back up to the QEMU main loop. In system emulation
we use this to decide to schedule a different guest CPU in SMP
configurations. In usermode emulation there is nothing to do,
so just ignore it and resume the guest.

This prevents an abort with "unhandled CPU exception 0x10004"
if the guest process uses the YIELD instruction.

Reported-by: Hunter Laux <hunterlaux@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1456833171-31900-1-git-send-email-peter.maydell@linaro.org
