Add a method to share Config across machines (#2608)

With `Module::{serialize,deserialize}` it should be possible to share
wasmtime modules across machines or CPUs. Serialization, however, embeds
a hash of all configuration values, including cranelift compilation
settings. By default wasmtime's selection of the native ISA would enable
ISA flags according to CPU features available on the host, but the same
CPU features may not be available across two machines.

This commit adds a `Config::cranelift_clear_cpu_flags` method which
allows clearing the target-specific ISA flags that are automatically
inferred by default for the native CPU. Options can then be
incrementally built back up as-desired with teh `cranelift_other_flag`
method.

crates/jit/src/compiler.rs

@@ -182,14 +182,10 @@ impl Hash for Compiler {
         // misc tunables.
         strategy.hash(hasher);
         isa.triple().hash(hasher);
-        features.hash(hasher);
-        // TODO: if this `to_string()` is too expensive then we should upstream
-        // a native hashing ability of flags into cranelift itself, but
-        // compilation and/or cache loading is relatively expensive so seems
-        // unlikely.
-        isa.flags().to_string().hash(hasher);
+        isa.hash_all_flags(hasher);
         isa.frontend_config().hash(hasher);
         tunables.hash(hasher);
+        features.hash(hasher);
 
         // Catch accidental bugs of reusing across crate versions.
         env!("CARGO_PKG_VERSION").hash(hasher);