fuzzgen: Generate compiler flags (#5020)

* fuzzgen: Test compiler flags

* cranelift: Generate `all()` function for all enum flags

This allows a user to iterate all flags that exist.

* fuzzgen: Minimize regalloc_checker compiles

* fuzzgen: Limit the amount of test case inputs

* fuzzgen: Add egraphs flag

It's finally here! 🥳

* cranelift: Add fuzzing comment to settings

* fuzzgen: Add riscv64

* fuzzgen:  Unconditionally enable some flags

cranelift/codegen/meta/src/shared/settings.rs

@@ -364,5 +364,7 @@ pub(crate) fn define() -> SettingGroup {
         false,
     );
 
+    // When adding new settings please check if they can also be added
+    // in cranelift/fuzzgen/src/lib.rs for fuzzing.
     settings.build()
 }