Implement runtime checks for compilation settings (#3899)

* Implement runtime checks for compilation settings

This commit fills out a few FIXME annotations by implementing run-time
checks that when a `Module` is created it has compatible codegen
settings for the current host (as `Module` is proof of "this code can
run"). This is done by implementing new `Engine`-level methods which
validate compiler settings. These settings are validated on
`Module::new` as well as when loading serialized modules.

Settings are split into two categories, one for "shared" top-level
settings and one for ISA-specific settings. Both categories now have
allow-lists hardcoded into `Engine` which indicate the acceptable values
for each setting (if applicable). ISA-specific settings are checked with
the Rust standard library's `std::is_x86_feature_detected!` macro. Other
macros for other platforms are not stable at this time but can be added
here if necessary.

Closes #3897

* Fix fall-through logic to actually be correct

* Use a `OnceCell`, not an `AtomicBool`

* Fix some broken tests

crates/environ/src/compilation.rs

@@ -235,7 +235,7 @@ pub trait Compiler: Send + Sync {
 }
 
 /// Value of a configured setting for a [`Compiler`]
-#[derive(Serialize, Deserialize, Hash, Eq, PartialEq)]
+#[derive(Serialize, Deserialize, Hash, Eq, PartialEq, Debug)]
 pub enum FlagValue {
     /// Name of the value that has been configured for this setting.
     Enum(Cow<'static, str>),