target-arm: Split DISAS_YIELD from DISAS_WFE

Currently we use DISAS_WFE for both WFE and YIELD instructions.
This is functionally correct because at the moment both of them
are implemented as "yield this CPU back to the top level loop so
another CPU has a chance to run". However it's rather confusing
that YIELD ends up calling HELPER(wfe), and if we ever want to
implement real behaviour for WFE and SEV it's likely to trip us up.

Split out the yield codepath to use DISAS_YIELD and a new
HELPER(yield) function, and have HELPER(wfe) call HELPER(yield).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1435672316-3311-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
