[X89] Ignore -mtune=generic to fix failures some users are seeing after D85384

Some code bases out there pass -mtune=generic to clang. This would have
been ignored prior to D85384. Now it results in an error
because "generic" isn't recognized by isValidCPUName.

And if we let it go through to the backend as a tune
setting it would get the tune flags closer to i386 rather
than a modern CPU.

I plan to change what tune=generic does in the backend in
a future patch. And allow this in the frontend.
But this should be a quick fix for the error some users
are seeing.