Rework riscv -march and -mabi detection

Instead of adding cases for all the operating systems, Use target
architecture directly as ISA string, replacing "riscv" with "rv", and
detect floating point ABI based on support for the D and F extensions.

Fixes rust-lang#795

src/lib.rs

@@ -2368,29 +2368,42 @@ impl Build {
                     let mut parts = target.split('-');
                     if let Some(arch) = parts.next() {
                         let arch = &arch[5..];
+
+                        // Assume that "rv{arch}" is a valid RISC-V ISA string.
+                        // The compiler would error out otherwise, and we fix
+                        // that later.
+                        cmd.args.push(format!("-march=rv{arch}").into());
+
+                        // Detect single-letter extensions from `arch`, assuming
+                        // no version numbers and canonical order
+                        let single_letter = arch
+                            .split(['_', 'z', 's'])
+                            .next()
+                            // The arch string starts with 32 or 64
+                            .expect("arch string cannot be empty");
+
+                        let riscv_implements = |ext| single_letter.contains(ext);
+
+                        // Detect ABI to select based on de facto standard
+
+                        let float_abi = if riscv_implements("g") || riscv_implements("d") {
+                            // Implements "d" (double-float), use double-float ABI
+                            "d"
+                        } else if riscv_implements("f") {
+                            // Implements "f" (single-float), use single-float ABI
+                            "f"
+                        } else {
+                            // No floating support, use soft-float ABI
+                            ""
+                        };
+
                         if arch.starts_with("64") {
-                            if target.contains("linux")
-                                | target.contains("freebsd")
-                                | target.contains("netbsd")
-                                | target.contains("linux")
-                            {
-                                cmd.args.push(("-march=rv64gc").into());
-                                cmd.args.push("-mabi=lp64d".into());
-                            } else {
-                                cmd.args.push(("-march=rv".to_owned() + arch).into());
-                                cmd.args.push("-mabi=lp64".into());
-                            }
-                        } else if arch.starts_with("32") {
-                            if target.contains("linux") {
-                                cmd.args.push(("-march=rv32gc").into());
-                                cmd.args.push("-mabi=ilp32d".into());
-                            } else {
-                                cmd.args.push(("-march=rv".to_owned() + arch).into());
-                                cmd.args.push("-mabi=ilp32".into());
-                            }
+                            cmd.args.push(format!("-mabi=lp64{float_abi}").into());
                         } else {
-                            cmd.args.push("-mcmodel=medany".into());
+                            cmd.args.push(format!("-mabi=ilp32{float_abi}").into());
                         }
+
+                        cmd.args.push("-mcmodel=medany".into());
                     }
                 }
             }