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
@@ -2171,6 +2171,7 @@ impl Build {
                             if target.contains("linux")
                                 | target.contains("freebsd")
                                 | target.contains("netbsd")
+                                | target.contains("openbsd")
                                 | target.contains("linux")
                             {
                                 cmd.args.push(("-march=rv64gc").into());
