Rework riscv -march and -mabi detection

Currently all the riscv*gc targets use the 'd' double-float ABI, and
soft-float otherwise. There's no need to detect the operating system
type. Fixes rust-lang#795.

src/lib.rs

@@ -1958,25 +1958,19 @@ impl Build {
                     let mut parts = target.split('-');
                     if let Some(arch) = parts.next() {
                         let arch = &arch[5..];
-                        if target.contains("linux") && arch.starts_with("64") {
-                            cmd.args.push(("-march=rv64gc").into());
-                            cmd.args.push("-mabi=lp64d".into());
-                        } else if target.contains("freebsd") && arch.starts_with("64") {
-                            cmd.args.push(("-march=rv64gc").into());
-                            cmd.args.push("-mabi=lp64d".into());
-                        } else if target.contains("openbsd") && arch.starts_with("64") {
-                            cmd.args.push(("-march=rv64gc").into());
-                            cmd.args.push("-mabi=lp64d".into());
-                        } else if target.contains("linux") && arch.starts_with("32") {
-                            cmd.args.push(("-march=rv32gc").into());
-                            cmd.args.push("-mabi=ilp32d".into());
-                        } else if arch.starts_with("64") {
-                            cmd.args.push(("-march=rv".to_owned() + arch).into());
-                            cmd.args.push("-mabi=lp64".into());
+
+                        cmd.args.push(("-march=rv".to_owned() + arch).into());
+
+                        // If riscv32gc-* or riscv64gc-* then double-float ABI,
+                        // otherwise use soft-float ABI.
+                        let float_abi = if arch.contains("g") { "d" } else { "" };
+
+                        if arch.starts_with("64") {
+                            cmd.args.push(("-mabi=lp64".to_owned() + float_abi).into());
                         } else {
-                            cmd.args.push(("-march=rv".to_owned() + arch).into());
-                            cmd.args.push("-mabi=ilp32".into());
+                            cmd.args.push(("-mabi=ilp32".to_owned() + float_abi).into());
                         }
+
                         cmd.args.push("-mcmodel=medany".into());
                     }
                 }