Use sigsetjmp instead of setjmp (#2645)

Apparently on macOS `setjmp` manipulates the process-wide signal mask
which adds a good deal of overhead. We don't actually need this
functionality so this commit switches to using the `sig` version of
setjmp/longjmp where we can explicitly ask the signal mask to not get
preserved. This came out of poking around on #2644 and on macOS locally
thi sdropped the overhead from 721ns to 55ns.

crates/runtime/build.rs

@@ -1,7 +1,13 @@
+use std::env;
+
 fn main() {
     println!("cargo:rerun-if-changed=src/helpers.c");
     cc::Build::new()
         .warnings(true)
+        .define(
+            &format!("CFG_TARGET_OS_{}", env::var("CARGO_CFG_TARGET_OS").unwrap()),
+            None,
+        )
         .file("src/helpers.c")
         .compile("helpers");
 }