rustbuild: Tweak CFLAGS to various libstd pieces

* Pass `opt_level(2)` when calculating CFLAGS to get the right flags on iOS
* Unconditionally pass `-O2` when compiling libbacktrace

This should...

Close #48903
Close #48906

Author: alexcrichton

src/bootstrap/cc_detect.rs

@@ -77,7 +77,7 @@ pub fn find(build: &mut Build) {
                                .collect::<HashSet<_>>();
     for target in targets.into_iter() {
         let mut cfg = cc::Build::new();
-        cfg.cargo_metadata(false).opt_level(0).warnings(false).debug(false)
+        cfg.cargo_metadata(false).opt_level(2).warnings(false).debug(false)
            .target(&target).host(&build.build);
         if target.contains("msvc") {
             cfg.static_crt(true);
@@ -109,7 +109,7 @@ pub fn find(build: &mut Build) {
     let hosts = build.hosts.iter().cloned().chain(iter::once(build.build)).collect::<HashSet<_>>();
     for host in hosts.into_iter() {
         let mut cfg = cc::Build::new();
-        cfg.cargo_metadata(false).opt_level(0).warnings(false).debug(false).cpp(true)
+        cfg.cargo_metadata(false).opt_level(2).warnings(false).debug(false).cpp(true)
            .target(&host).host(&build.build);
         let config = build.config.target_config.get(&host);
         if let Some(cxx) = config.and_then(|c| c.cxx.as_ref()) {

src/libstd/build.rs

@@ -98,7 +98,7 @@ fn build_libbacktrace(host: &str, target: &str) -> Result<(), ()> {
                 .arg("--disable-host-shared")
                 .arg(format!("--host={}", build_helper::gnu_target(target)))
                 .arg(format!("--build={}", build_helper::gnu_target(host)))
-                .env("CFLAGS", env::var("CFLAGS").unwrap_or_default() + " -fvisibility=hidden"));
+                .env("CFLAGS", env::var("CFLAGS").unwrap_or_default() + " -fvisibility=hidden -O2"));
 
     run(Command::new(build_helper::make(host))
                 .current_dir(&native.out_dir)