When the `WEVAL` option is turned on (`cmake -DWEVAL=ON`), this PR adds:

- Integration to the CMake machinery to fetch a PBL+weval-ified version
  of SpiderMonkey artifacts;
- Likewise, to fetch weval binaries;
- A rule to pre-build a compilation cache of IC bodies specific to the
  StarlingMonkey build, so weval can use this cache and spend time only
  on user-provided code on first run;
- Integration in `componentize.sh`.

When built with:

```
$ mkdir build/; cd build/
$ cmake .. -DCMAKE_BUILD_TYPE=Release -DUSE_WASM_OPT=OFF -DWEVAL=ON
$ make
```

We can then do:

```
$ build/componentize.sh file.js --aot -o file.wasm
$ wasmtime serve -S cli=y file.wasm
```

Using the Richards Octane benchmark adapted slightly with a `main()` for
the HTTP server world [1], I get the following results:

```
%  build/componentize.sh
richards.js --aot -o weval.wasm
Componentizing richards.js into weval.wasm
[ verbose weval progress output ]

% wasmtime serve -S cli=y weval.wasm
Serving HTTP on http://0.0.0.0:8080/
stdout [0] :: Log: Richards: 676
stdout [0] :: Log: ----
stdout [0] :: Log: Score (version 9): 676

% wasmtime serve -S cli=y base.wasm
Serving HTTP on http://0.0.0.0:8080/
stdout [0] :: Log: Richards: 189
stdout [0] :: Log: ----
stdout [0] :: Log: Score (version 9): 189
```

[1]: https://gist.github.com/cfallin/4b18da12413e93f7e88568a92d09e4b7

This commit updates to weval v0.2.7 which has no output by default,
allowing the expected-failure tests checking syntax error messages to
pass; we now pass 9/10 integration tests.

It also updates the flags on `componentize.sh`: a `--verbose` flag to
allow the user to see weval progress messages (perhaps useful if the
build is taking a long time, or to see the stats on function
specializations); and removal of the `--aot` flag, because there is only
one valid setting for a build configuration and it is not baked into the
shell script automatically.

…idermonkey-wasi-embedding#19).