Move trap handler initialization to per-Store (#1644)

Previously we initialized trap handling (signals/etc) once-per-instance
but that's a bit too granular since we only need to do this as
one-time per-program initialization. This moves the initialization to
`Store` instead which means that we'll call this at least once per
thread, which some platforms may need (none currently do, they all only
need per-program initialization, but Fuchsia will need per-thread
initialization).

crates/api/src/runtime.rs

@@ -743,6 +743,13 @@ pub(crate) struct StoreInner {
 impl Store {
     /// Creates a new store to be associated with the given [`Engine`].
     pub fn new(engine: &Engine) -> Store {
+        // Ensure that wasmtime_runtime's signal handlers are configured. Note
+        // that at the `Store` level it means we should perform this
+        // once-per-thread. Platforms like Unix, however, only require this
+        // once-per-program. In any case this is safe to call many times and
+        // each one that's not relevant just won't do anything.
+        wasmtime_runtime::init_traps();
+
         let isa = native::builder().finish(settings::Flags::new(engine.config.flags.clone()));
         let compiler = Compiler::new(
             isa,