Defaulting NativeAOT to current machines CPU features doesn't represent a real usecase scenario

Defaulting to current machines features, introduced in #1994, doesn't seem reasonable to me, since a developer would likely never distribute such build to customers, ignoring servers with known hardware.

As @tannergooding said a few times on the C# discord, comparing a JIT with an AOT using `march=native` isn't meaningful and here we're doing the same.

I'd introduce a separate configuration option `useNativeCpuFeatures` and default that to false to make benchmarking a bit more reasonable.