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.

crates/environ/src/data_structures.rs

@@ -10,11 +10,13 @@ pub mod ir {
 }
 
 pub mod settings {
-    pub use cranelift_codegen::settings::{builder, Builder, Configurable, Flags};
+    pub use cranelift_codegen::settings::{builder, Builder, Configurable, Flags, SetError};
 }
 
 pub mod isa {
-    pub use cranelift_codegen::isa::{unwind, CallConv, RegUnit, TargetFrontendConfig, TargetIsa};
+    pub use cranelift_codegen::isa::{
+        unwind, Builder, CallConv, RegUnit, TargetFrontendConfig, TargetIsa,
+    };
 }
 
 pub mod entity {