fuzzgen: Fuzz ISA flags (#6001)

fuzz/fuzz_targets/cranelift-fuzzgen.rs

@@ -179,9 +179,10 @@ impl TestCase {
 
         // TestCase is meant to be consumed by a runner, so we make the assumption here that we're
         // generating a TargetIsa for the host.
-        let builder =
+        let mut builder =
             builder_with_options(true).expect("Unable to build a TargetIsa for the current host");
         let flags = gen.generate_flags(builder.triple().architecture)?;
+        gen.set_isa_flags(&mut builder, IsaFlagGen::Host)?;
         let isa = builder.finish(flags)?;
 
         // When generating functions, we allow each function to call any function that has

fuzz/fuzz_targets/cranelift-icache.rs

@@ -48,13 +48,15 @@ impl FunctionWithIsa {
         // a supported one, so that the same fuzz input works across different build
         // configurations.
         let target = u.choose(isa::ALL_ARCHITECTURES)?;
-        let builder = isa::lookup_by_name(target).map_err(|_| arbitrary::Error::IncorrectFormat)?;
+        let mut builder =
+            isa::lookup_by_name(target).map_err(|_| arbitrary::Error::IncorrectFormat)?;
         let architecture = builder.triple().architecture;
 
         let mut gen = FuzzGen::new(u);
         let flags = gen
             .generate_flags(architecture)
             .map_err(|_| arbitrary::Error::IncorrectFormat)?;
+        gen.set_isa_flags(&mut builder, IsaFlagGen::All)?;
         let isa = builder
             .finish(flags)
             .map_err(|_| arbitrary::Error::IncorrectFormat)?;