iOS: NEON detection is broken

[Good build: Sep 5-6](http://pullrequest.opencv.org/buildbot/builders/3_4_iOS-mac/builds/146)
[Broken build: Sep 6-7](http://pullrequest.opencv.org/buildbot/builders/3_4_iOS-mac/builds/147):

```
--   CPU/HW features:
--     Baseline:
--       requested:                 NEON FP16
--       disabled:                  VFPV3 NEON
```