Support riscv64-unknown-openbsd

Index: third_party/rust/cc/src/lib.rs
--- third_party/rust/cc/src/lib.rs.orig
+++ third_party/rust/cc/src/lib.rs
@@ -1781,7 +1781,7 @@ 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") {
+                        if (target.contains("linux") || 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") {
