Add more flexible method for specifying jit instrinsics

[AndyAyersMS]

Per discussion in dotnet/coreclr#13748.

Add ability to mark a method with an [Intrinsic] attribute and pass this through to the jit via the method flags. Jit can then query back to the EE and match on class/name to figure out exactly what intrinsic this is, rather than using the old system of predefined intrinsic IDs.

Will initially use this for Enum.HasFlag support.

[AndyAyersMS]

Fixed via dotnet/coreclr#13815.