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.

src/commands/settings.rs

@@ -62,7 +62,7 @@ impl SettingsCommand {
         }
 
         if self.target.is_none() {
-            let compiler = builder.build();
+            let compiler = builder.build()?;
             println!();
             println!("Settings inferred for the current host:");