[X86] Change the scheduler model for 'pentium4' to SandyBridgeModel.

I meant to do this in D83913, but missed it while updating the
feature list.

Interestingly I think this is disabling the postRA scheduler. But
it does match our default 64-bit behavior.

Reviewed By: echristo

Differential Revision: https://reviews.llvm.org/D83996