[X86] Copy the tuning features and scheduler model from pentium4/x86-64 to generic

This is preparation for making clang default to -mtune=generic when no -march is specified. This will allow the default tuning to be "generic" even though our default march is "pentium4" or "x86-64".

To avoid llc lit test regressions, if no mcpu is specified, I've defaulted tune to use i586 to match the old tuning settings of no CPU. Some tests explicitly used -mcpu=generic which I've removed so they instead get this default of architecture features from generic and tune from i586.

I updated one llvm-mca test to check a different CPU since generic has a scheduler model now

Differential Revision: https://reviews.llvm.org/D86312