[NFC][RISCV] Enable TuneNoDefaultUnroll feature to control targets which use...

[NFC][RISCV] Enable TuneNoDefaultUnroll feature to control targets which use default unroll preference

In RISCVTargetTransformInfo, enumerating the processor family is not a good way to predict.
Because it needs to enumerate many subtarget family and is hard to update if add new subtarget.
Instead, create a feature to distinguish whether targets want to use default unroll preference or not.

Keep TuneSiFive7 because it's flag to indicate subtarget family, which may used in other place.

Differential Revision: https://reviews.llvm.org/D125741