[RISCV] Add RISCVProcFamilyEnum and add SiFive7.

Use it to remove explicit string compares from unrolling preferences.

I'm of two minds on this. Ideally, we would define things in terms
of architectural or microarchitectural features, but it's hard to
do that with things like unrolling preferences without just ending up
with FeatureSiFive7UnrollingPreferences.

Having a proc enum is consistent with ARM and AArch64. X86 only has
a few and is trying to move away from it.

Reviewed By: asb, mcberg2021

Differential Revision: https://reviews.llvm.org/D117060