Support riscv64gc-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
@@ -2392,7 +2392,7 @@ impl Build {
                     // get the 32i/32imac/32imc/64gc/64imac/... part
                     let arch = &target.full_arch[5..];
                     if arch.starts_with("64") {
-                        if matches!(target.os, "linux" | "freebsd" | "netbsd") {
+                        if matches!(target.os, "linux" | "freebsd" | "netbsd" | "openbsd") {
                             cmd.args.push(("-march=rv64gc").into());
                             cmd.args.push("-mabi=lp64d".into());
                         } else {
