Add trampoline compilation support for lowered imports (#4206)

* Add trampoline compilation support for lowered imports

This commit adds support to the component model implementation for
compiling trampolines suitable for calling host imports. Currently this
is purely just the compilation side of things, modifying the
wasmtime-cranelift crate and additionally filling out a new
`VMComponentOffsets` type (similar to `VMOffsets`). The actual creation
of a `VMComponentContext` is still not performed and will be a
subsequent PR.

Internally though some tests are actually possible with this where we at
least assert that compilation of a component and creation of everything
in-memory doesn't panic or trip any assertions, so some tests are added
here for that as well.

* Fix some test errors

crates/cranelift/src/obj.rs

@@ -88,7 +88,7 @@ impl<'a> ModuleTextBuilder<'a> {
     ///
     /// Returns the symbol associated with the function as well as the range
     /// that the function resides within the text section.
-    fn append_func(
+    pub fn append_func(
         &mut self,
         labeled: bool,
         name: Vec<u8>,