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.

cranelift/codegen/src/isa/mod.rs

@@ -69,6 +69,7 @@ use alloc::boxed::Box;
 use core::any::Any;
 use core::fmt;
 use core::fmt::{Debug, Formatter};
+use core::hash::Hasher;
 use target_lexicon::{triple, Architecture, PointerWidth, Triple};
 use thiserror::Error;
 
@@ -265,6 +266,10 @@ pub trait TargetIsa: fmt::Display + Send + Sync {
     /// Get the ISA-independent flags that were used to make this trait object.
     fn flags(&self) -> &settings::Flags;
 
+    /// Hashes all flags, both ISA-independent and ISA-specific, into the
+    /// specified hasher.
+    fn hash_all_flags(&self, hasher: &mut dyn Hasher);
+
     /// Get the default calling convention of this target.
     fn default_call_conv(&self) -> CallConv {
         CallConv::triple_default(self.triple())