Use -O3 instead of -O (#2070)

As recommended in
https://www.reddit.com/r/rust/comments/balb45/why_is_hypers_max_latency_so_high_in_deno/ekck1ww/

build_extra/rust/rust.gni

@@ -209,7 +209,10 @@ template("rust_crate") {
       args += [ "-g" ]
     }
     if (is_official_build) {
-      args += [ "-O" ]
+      args += [
+        "-C",
+        "opt-level=3",
+      ]
     }
     if (is_test) {
       args += [ "--test" ]