Use qCpuHasFeature because the check can be optimized out of existence

qCpuHasFeature also checks the compiler flags of this particular build.
If the feature was enabled by the compiler in this .cpp, then no runtime
check is performed and the test becomes an unconditional true.

Change-Id: Ibb7d333e59a0f4ed06b7d0056547177fd69658a0
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>