std_detect: RISC-V: Best effort implication by target config (C-related)

On the RISC-V architecture, there are some complex implications around the
"C" extension.  This commit implements implication of such target features
in a best effort manner.

The reason why we can make this exact is lack of `"zcd"` and `"zcf"`
target features (that would require additional feature handling system).

Still, weaker implications implemented here are formally verified to be
optimal if we have to avoid tautology (except `"c"`) and uses of `"zcf"`
and `"zcd"` are prohibited.

For "C", the author deduced minimal expression which makes equal to the
"C" extension and extracted all cases where `"zcd"` and `"zcf"` are not
involved.

For "Zcd" and "Zcf", the author created proof statements, each stating
that implication expression (which avoids tautology and accidentally
avoids using `"zcd"` or `"zcf"`) is optimal to force corresponding
target feature to be enabled.

Author: a4lg

library/std_detect/src/detect/arch/riscv.rs

@@ -270,14 +270,22 @@ features! {
     /// "Zhinxmin" Extension for Minimal Half-Precision Floating-Point in Integer Registers
 
     @FEATURE: #[stable(feature = "riscv_ratified", since = "1.78.0")] c: "c";
+    implied by cfg(any(
+        target_feature = "c",
+        all(
+            not(target_feature = "d"),
+            any(not(target_arch = "riscv32"), not(target_feature = "f")),
+            target_feature = "zca",
+        )
+    ));
     /// "C" Extension for Compressed Instructions
     @FEATURE: #[unstable(feature = "stdarch_riscv_feature_detection", issue = "111192")] zca: "zca";
     /// "Zca" Compressed Instructions excluding Floating-Point Loads/Stores
     @FEATURE: #[unstable(feature = "stdarch_riscv_feature_detection", issue = "111192")] zcf: "zcf";
-    without cfg check: true;
+    implied by cfg(all(target_arch = "riscv32", target_feature = "c", target_feature = "f"));
     /// "Zcf" Compressed Instructions for Single-Precision Floating-Point Loads/Stores on RV32
     @FEATURE: #[unstable(feature = "stdarch_riscv_feature_detection", issue = "111192")] zcd: "zcd";
-    without cfg check: true;
+    implied by cfg(all(target_feature = "c", target_feature = "d"));
     /// "Zcd" Compressed Instructions for Double-Precision Floating-Point Loads/Stores
     @FEATURE: #[unstable(feature = "stdarch_riscv_feature_detection", issue = "111192")] zcb: "zcb";
     /// "Zcb" Simple Code-size Saving Compressed Instructions