Fuzzing: add use_egraphs option back to fuzzing config generator. (#5388)

This PR reverts #5128 (commit b3333bf9ea),
adding back the ability for the fuzzing config generator to set
the `use_egraphs` Cranelift option. This will start to fuzz the
egraphs-based optimization framework again, now that #5382 has landed.

cranelift/fuzzgen/src/lib.rs

@@ -253,6 +253,7 @@ where
             "enable_incremental_compilation_cache_checks",
             "regalloc_checker",
             "enable_llvm_abi_extensions",
+            "use_egraphs",
         ];
         for flag_name in bool_settings {
             let enabled = self