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/wasmtime/Cargo.toml

@@ -81,3 +81,8 @@ uffd = ["wasmtime-runtime/uffd"]
 
 # Enables support for all architectures in JIT and the `wasmtime compile` CLI command.
 all-arch = ["wasmtime-jit/all-arch"]
+
+# Enables trap handling using POSIX signals instead of Mach exceptions on MacOS.
+# It is useful for applications that do not bind their own exception ports and
+# need portable signal handling.
+posix-signals-on-macos = ["wasmtime-runtime/posix-signals-on-macos"]

crates/wasmtime/src/lib.rs

@@ -401,7 +401,7 @@ pub use crate::types::*;
 pub use crate::values::*;
 
 cfg_if::cfg_if! {
-    if #[cfg(target_os = "macos")] {
+    if #[cfg(all(target_os = "macos", not(feature = "posix-signals-on-macos")))] {
         // no extensions for macOS at this time
     } else if #[cfg(unix)] {
         pub mod unix;