cmpbge emulation improvements

# gpg: Signature made Wed 02 Sep 2015 20:25:10 BST using RSA key ID 4DD0279B
# gpg: Good signature from "Richard Henderson <rth7680@gmail.com>"
# gpg:                 aka "Richard Henderson <rth@redhat.com>"
# gpg:                 aka "Richard Henderson <rth@twiddle.net>"

* remotes/rth/tags/pull-axp-20150902:
  target-alpha: Special case cmpbge with zero
  target-alpha: Rewrite helper_cmpbge using bit tests

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>