rustc_target: Adjust RISC-V feature implication

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 referred 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 - "clmulr" instruction == Zbkc)
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.
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).

compiler/rustc_target/src/target_features.rs

@@ -516,7 +516,7 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zawrs", Unstable(sym::riscv_target_feature), &[]),
     ("zba", Stable, &[]),
     ("zbb", Stable, &[]),
-    ("zbc", Stable, &[]),
+    ("zbc", Stable, &["zbkc"]), // Zbc ⊃ Zbkc
     ("zbkb", Stable, &[]),
     ("zbkc", Stable, &[]),
     ("zbkx", Stable, &[]),
@@ -545,20 +545,20 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zknd", Stable, &[]),
     ("zkne", Stable, &[]),
     ("zknh", Stable, &[]),
-    ("zkr", Stable, &["zicsr"]),
+    ("zkr", Stable, &[]),
     ("zks", Stable, &["zbkb", "zbkc", "zbkx", "zksed", "zksh"]),
     ("zksed", Stable, &[]),
     ("zksh", Stable, &[]),
     ("zkt", Stable, &[]),
     ("ztso", Unstable(sym::riscv_target_feature), &[]),
-    ("zvbb", Unstable(sym::riscv_target_feature), &["zvkb"]),
+    ("zvbb", Unstable(sym::riscv_target_feature), &["zvkb"]), // Zvbb ⊃ Zvkb
     ("zvbc", Unstable(sym::riscv_target_feature), &["zve64x"]),
     ("zve32f", Unstable(sym::riscv_target_feature), &["zve32x", "f"]),
     ("zve32x", Unstable(sym::riscv_target_feature), &["zvl32b", "zicsr"]),
     ("zve64d", Unstable(sym::riscv_target_feature), &["zve64f", "d"]),
     ("zve64f", Unstable(sym::riscv_target_feature), &["zve32f", "zve64x"]),
     ("zve64x", Unstable(sym::riscv_target_feature), &["zve32x", "zvl64b"]),
-    ("zvfh", Unstable(sym::riscv_target_feature), &["zvfhmin", "zfhmin"]),
+    ("zvfh", Unstable(sym::riscv_target_feature), &["zvfhmin", "zve32f", "zfhmin"]), // Zvfh ⊃ Zvfhmin
     ("zvfhmin", Unstable(sym::riscv_target_feature), &["zve32f"]),
     ("zvkb", Unstable(sym::riscv_target_feature), &["zve32x"]),
     ("zvkg", Unstable(sym::riscv_target_feature), &["zve32x"]),
@@ -567,7 +567,7 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zvkned", Unstable(sym::riscv_target_feature), &["zve32x"]),
     ("zvkng", Unstable(sym::riscv_target_feature), &["zvkn", "zvkg"]),
     ("zvknha", Unstable(sym::riscv_target_feature), &["zve32x"]),
-    ("zvknhb", Unstable(sym::riscv_target_feature), &["zve64x"]),
+    ("zvknhb", Unstable(sym::riscv_target_feature), &["zvknha", "zve64x"]), // Zvknhb ⊃ Zvknha
     ("zvks", Unstable(sym::riscv_target_feature), &["zvksed", "zvksh", "zvkb", "zvkt"]),
     ("zvksc", Unstable(sym::riscv_target_feature), &["zvks", "zvbc"]),
     ("zvksed", Unstable(sym::riscv_target_feature), &["zve32x"]),