Remove dependency on TargetIsa from Wasmtime crates (#3178)

This commit started off by deleting the `cranelift_codegen::settings`
reexport in the `wasmtime-environ` crate and then basically played
whack-a-mole until everything compiled again. The main result of this is
that the `wasmtime-*` family of crates have generally less of a
dependency on the `TargetIsa` trait and type from Cranelift. While the
dependency isn't entirely severed yet this is at least a significant
start.

This commit is intended to be largely refactorings, no functional
changes are intended here. The refactorings are:

* A `CompilerBuilder` trait has been added to `wasmtime_environ` which
  server as an abstraction used to create compilers and configure them
  in a uniform fashion. The `wasmtime::Config` type now uses this
  instead of cranelift-specific settings. The `wasmtime-jit` crate
  exports the ability to create a compiler builder from a
  `CompilationStrategy`, which only works for Cranelift right now. In a
  cranelift-less build of Wasmtime this is expected to return a trait
  object that fails all requests to compile.

* The `Compiler` trait in the `wasmtime_environ` crate has been souped
  up with a number of methods that Wasmtime and other crates needed.

* The `wasmtime-debug` crate is now moved entirely behind the
  `wasmtime-cranelift` crate.

* The `wasmtime-cranelift` crate is now only depended on by the
  `wasmtime-jit` crate.

* Wasm types in `cranelift-wasm` no longer contain their IR type,
  instead they only contain the `WasmType`. This is required to get
  everything to align correctly but will also be required in a future
  refactoring where the types used by `cranelift-wasm` will be extracted
  to a separate crate.

* I moved around a fair bit of code in `wasmtime-cranelift`.

* Some gdb-specific jit-specific code has moved from `wasmtime-debug` to
  `wasmtime-jit`.

crates/jit/src/code_memory.rs

@@ -5,6 +5,7 @@ use crate::object::{
     ObjectUnwindInfo,
 };
 use crate::unwind::UnwindRegistry;
+use crate::Compiler;
 use anyhow::{Context, Result};
 use object::read::{File as ObjectFile, Object, ObjectSection, ObjectSymbol};
 use region;
@@ -12,7 +13,7 @@ use std::collections::BTreeMap;
 use std::mem::ManuallyDrop;
 use std::{cmp, mem};
 use wasmtime_environ::{
-    isa::{unwind::UnwindInfo, TargetIsa},
+    isa::unwind::UnwindInfo,
     wasm::{FuncIndex, SignatureIndex},
     CompiledFunction,
 };
@@ -131,7 +132,7 @@ impl CodeMemory {
     }
 
     /// Make all allocated memory executable.
-    pub fn publish(&mut self, isa: &dyn TargetIsa) {
+    pub fn publish(&mut self, compiler: &Compiler) {
         self.push_current(0)
             .expect("failed to push current memory map");
 
@@ -142,7 +143,7 @@ impl CodeMemory {
         } in &mut self.entries[self.published..]
         {
             // Remove write access to the pages due to the relocation fixups.
-            r.publish(isa)
+            r.publish(compiler)
                 .expect("failed to publish function unwind registry");
 
             if !m.is_empty() {