Restore POSIX signal handling on MacOS behind a feature flag (#3063)

* Restore POSIX signal handling on MacOS behind a feature flag

As described in Issue #3052, the switch to Mach Exception handling
removed `unix::StoreExt` from the public API of crate on MacOS.
That is a breaking change and makes it difficult for some
application to upgrade to the current stable Wasmtime.

As a workaround this PR introduces a feature flag called
`posix-signals-on-macos` that restores the old behaviour on MacOS.
The flag is disabled by default.

* Fix test guard

* Fix formatting in the test

crates/runtime/src/traphandlers.rs

@@ -27,7 +27,7 @@ extern "C" {
 }
 
 cfg_if::cfg_if! {
-    if #[cfg(target_os = "macos")] {
+    if #[cfg(all(target_os = "macos", not(feature = "posix-signals-on-macos")))] {
         mod macos;
         use macos as sys;
     } else if #[cfg(unix)] {