FilterNamedIntrinsicMethodAttribs checks className for unsupported isas not classname

[davidwrighton]

As noted by @saucecontrol in #34550 (comment)_ crossgen doesn't correctly check for isaName instead of className which checking for the use of the crossgen2 corelib unsupported isas like Avx, Avx2, etc. We should fix this as it might cause very odd behavior for code in corelib which uses such logic.

[tannergooding]

This looks like an oversight on my part when I changed the logic in dotnet/coreclr#25365