rustc_target: Adjust RISC-V feature implication

[a4lg]

This commit adjusts feature implication of the RISC-V ISA for better feature detection from the user perspective.

The main rule is:

• If the feature A is a functional superset of the feature B (A ⊃ B),
  A is to imply B, even if this implication is not on the manual.

Such implications (not directly written in the ISA manual) are commented as A ⊃ B
which means "A is a (functional) superset of B".

1. Zbc → Zbkc (add as a superset)
   The Zbkc extension is a subset of the Zbc extension (Zbc minus clmulr instruction).
2. Zkr → (nothing) (remove dependency to Zicsr)
   Implication to the Zicsr extension is removed because (although nearly harmless), the Zkr extension (or the seed CSR section) defines its own subset of the Zicsr extension (guaranteed to work against the seed CSR which needs read/write access).
3. Zvbb → Zvkb (comment as a superset)
   This implication was already there but not denoted as a functional superset. This commit adds the comment.
4. Zvfh → Zvfhmin (comment as a superset)
   This is similar to the case above (Zvbb → Zvkb).
5. Zvfh → Zve32f (add implication per the ISA specification)
   This dependency is on the ISA manual but was missing (due to the fact that Zvfh indirectly implies Zve32f on the current implementation through Zvfh → Zvfhmin which is a functional relation). This commit ensures that this is also ISA-compliant in the source code level (there's no functional changes though).
6. Zvknhb → Zvknha (add as a superset)
   The Zvknhb extension (SHA-256 / SHA-512) is a functional superset of the Zvknha extension (SHA-256 only).

---

Related:

• Tracking issue for RFC 2045: improving #[target_feature] #44839
  (riscv_target_feature)
• RISC-V: riscv_hwprobe-based feature detection on Linux / Android stdarch#1770
  (synchronized with stdarch's latest RISC-V implication logic)

@rustbot r? @Amanieu
@rustbot label +T-compiler +O-riscv +A-target-feature