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

tests/all/custom_signal_handler.rs

@@ -1,4 +1,7 @@
-#[cfg(target_os = "linux")]
+#[cfg(any(
+    target_os = "linux",
+    all(target_os = "macos", feature = "posix-signals-on-macos")
+))]
 mod tests {
     use anyhow::Result;
     use std::sync::atomic::{AtomicBool, Ordering};