Use higher level APIs for getting the number of cores if WelsCPUFeatureDetect didn't report anything

On processors without HTT, WelsCPUFeatureDetect can't return
a number of cores but might still return a nonzero set of
CPU feature flags. Previously the nonzero cpu feature flag
indicated that cpuid worked and the encoder wouldn't use the
higher level API for getting the number of cores, even though the
number of cores was left at 1.
Merge pull request #361 from mstorsjo/get-core-count

Use higher level APIs for getting the number of cores if WelsCPUFeatureDetect didn't report anything