Move cranelift dependencies to wasmtime-environ (#669)

Groups all CL data structures into single dependency to be used accross wasmtime project.

crates/jit/src/native.rs

@@ -0,0 +1,14 @@
+#![allow(missing_docs)]
+
+use cranelift_codegen;
+
+pub fn builder() -> cranelift_codegen::isa::Builder {
+    cranelift_native::builder().expect("host machine is not a supported target")
+}
+
+pub fn call_conv() -> cranelift_codegen::isa::CallConv {
+    use target_lexicon::HOST;
+    cranelift_codegen::isa::CallConv::triple_default(&HOST)
+}
+
+pub use cranelift_codegen::isa::lookup;