Stabilize Ratified RISC-V Target Features

As shortly discussed on Zulip
(https://rust-lang.zulipchat.com/#narrow/stream/250483-t-compiler.2Frisc-v/topic/Stabilization.20of.20RISC-V.20Target.20Features/near/394793704),
this commit stabilizes the ratified RISC-V instruction bases and
extensions.

Specifically, this commit stabilizes the:
* Atomic Instructions (A) on v2.0
* Compressed Instructions (C) on v2.0
* Integer Multiplication and Division (M) on v2.0
* Bit Manipulations (B) on v1.0 listed as `zba`, `zbc`, `zbs`
* Scalar Cryptography (Zk) v1.0.1 listed as `zk`, `zkn`, `zknd`, `zkne`, `zknh`, `zkr`, `zks`, `zksed`, `zksh`, `zkt`, `zbkb`, `zbkc` `zkbx`