Support disabling of extensions

To test all code paths of apps that use cpu_features to select between different implementations of an algorithm, it would be useful to be able to mock having a CPU with fewer features than actually supported.

While this can be done in wrapper layer which uses cpu_features to query the real feature set, little prevents direct use of cpu_features functions that would break the purpose of the wrapper. Hence I think it would be useful if this was a core feature of cpu_features itself.

One complication is that for tests to be able to run concurrently this would have to be thread-local (assuming the tests are single-threaded). For multi-threaded tests a global mutex could be used to prevent concurrent execution from the point where cpu_features is provided with override options, to the point where they're removed.

Multiple users of cpu_features have to deal with this themselves currently, so having it as a core feature instead avoids duplicate effort and incompatible solutions which might have subtle bugs that are best fixed centrally.