Wasmtime: allow using the experimental Cranelift x64 backend in cli;

This introduces two changes:

- first, a Cargo feature is added to make it possible to use the
Cranelift x64 backend directly from wasmtime's CLI.
- second, when passing a `cranelift-flags` parameter, and the given
parameter's name doesn't exist at the target-independent flag level, try
to set it as a target-dependent setting.

These two changes make it possible to try out the new x64 backend with:

    cargo run --features experimental_x64 -- run --cranelift-flags use_new_backend=true -- /path/to/a.wasm

Right now, this will fail because most opcodes required by the
trampolines are actually not implemented yet.

cranelift/codegen/src/isa/mod.rs

@@ -150,6 +150,7 @@ pub enum LookupError {
 
 /// Builder for a `TargetIsa`.
 /// Modify the ISA-specific settings before creating the `TargetIsa` trait object with `finish`.
+#[derive(Clone)]
 pub struct Builder {
     triple: Triple,
     setup: settings::Builder,