Clean up a bit of the uses of CPU in the TargetInfo classes. This makes

it an error if a CPU is provided for a target that doesn't implement
logic handling CPU settings, to match the ABI settings. It also removes
the CPU parameter from the getDefaultFeatures method. This parameter was
always filled in with the same value as setCPU was called with, and at
this point every single target implementation that referenced the CPU
within this function has needed to store the CPU via setCPU anyways in
order to implement other interface points.

llvm-svn: 140683