Cranelift: ensure ISA level needed for SIMD is present when SIMD is enabled. (#3816)

Addresses #3809: when we are asked to create a Cranelift backend with
shared flags that indicate support for SIMD, we should check that the
ISA level needed for our SIMD lowerings is present.

crates/cranelift/src/builder.rs

@@ -101,12 +101,15 @@ impl CompilerBuilder for Builder {
         Ok(())
     }
 
-    fn build(&self) -> Box<dyn wasmtime_environ::Compiler> {
+    fn build(&self) -> Result<Box<dyn wasmtime_environ::Compiler>> {
         let isa = self
             .isa_flags
             .clone()
-            .finish(settings::Flags::new(self.flags.clone()));
-        Box::new(crate::compiler::Compiler::new(isa, self.linkopts.clone()))
+            .finish(settings::Flags::new(self.flags.clone()))?;
+        Ok(Box::new(crate::compiler::Compiler::new(
+            isa,
+            self.linkopts.clone(),
+        )))
     }
 
     fn settings(&self) -> Vec<Setting> {

crates/environ/src/compilation.rs

@@ -104,7 +104,7 @@ pub trait CompilerBuilder: Send + Sync + fmt::Debug {
     fn settings(&self) -> Vec<Setting>;
 
     /// Builds a new [`Compiler`] object from this configuration.
-    fn build(&self) -> Box<dyn Compiler>;
+    fn build(&self) -> Result<Box<dyn Compiler>>;
 }
 
 /// Description of compiler settings returned by [`CompilerBuilder::settings`].

crates/wasmtime/src/engine.rs

@@ -64,7 +64,7 @@ impl Engine {
         Ok(Engine {
             inner: Arc::new(EngineInner {
                 #[cfg(compiler)]
-                compiler: config.compiler.build(),
+                compiler: config.compiler.build()?,
                 config,
                 allocator,
                 signatures: registry,