Change how flags are stored in serialized modules.

This commit changes how both the shared flags and ISA flags are stored in the
serialized module to detect incompatibilities when a serialized module is
instantiated.

It improves the error reporting when a compiled module has mismatched shared
flags.

cranelift/codegen/src/isa/x64/settings.rs

@@ -1,6 +1,6 @@
 //! x86 Settings.
 
-use crate::settings::{self, detail, Builder};
+use crate::settings::{self, detail, Builder, Value};
 use core::fmt;
 
 // Include code generated by `cranelift-codegen/meta/src/gen_settings.rs:`. This file contains a