target-arm: convert add_cc and sub_cc helpers to TCG

target-arm: convert add_cc and sub_cc helpers to TCG

Now that the setcond TCG op is available, it's possible to replace
add_cc and sub_cc helpers by TCG code. The code generated by TCG is
actually very close to the one generated by GCC for the helper, and
this avoid all the consequences of using an helper: globals saved back
to memory, no possible optimization, call overhead, etc.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>