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/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,