Add a wasmtime settings command to print Cranelift settings.

This commit adds the `wasmtime settings` command to print out available
Cranelift settings for a target (defaults to the host).

The compile command has been updated to remove the Cranelift ISA options in
favor of encouraging users to use `wasmtime settings` to discover what settings
are available.  This will reduce the maintenance cost for syncing the compile
command with Cranelift ISA flags.

cranelift/codegen/src/isa/mod.rs

@@ -200,6 +200,16 @@ pub struct Builder {
 }
 
 impl Builder {
+    /// Gets the triple for the builder.
+    pub fn triple(&self) -> &Triple {
+        &self.triple
+    }
+
+    /// Iterates the available settings in the builder.
+    pub fn iter(&self) -> impl Iterator<Item = settings::Setting> {
+        self.setup.iter()
+    }
+
     /// Combine the ISA-specific settings with the provided ISA-independent settings and allocate a
     /// fully configured `TargetIsa` trait object.
     pub fn finish(self, shared_flags: settings::Flags) -> Box<dyn TargetIsa> {