Misc spidermonkey commands

perf integration

1. When running configure, pass the `--enable-perf` configuration option.  Re-build the shell.
2. Run the shell with

```
IONPERF=func perf record $jsshell --no-wasm-multi-value --shared-memory=off --wasm-compiler=cranelift /path/to/script.js
```

An ExecutableAllocator is leaked on shutdown, which causes an assertion to
trigger: `Assertion failure: m_refCount == 1, at /home/ben/code/mozilla-inbound/js/src/jit/ExecutableAllocator.cpp:51`

3. Perf mapping should be generated in /tmp as `perf-$PID.map`, and a `perf.data` file should have
been generated in the current working directory.  It can then be inspected with `perf report`, or
[HotSpot](https://github.com/KDAB/hotspot/), etc.

seeing generated code

1. Use a build with debug enabled (`--enable-debug`) or jit spew enabled (`--enable-jitspew`).
2. Run with `IONFLAGS=logs $jsshell /path/to/script > /tmp/logs 2>&1`. This generates a large log,
since it contains generated JIT code for JS too.
3. Open /tmp/logs and grep "wasm cranelift". You'll see `Start of wasm cranelift code for function
...`, and there's also something at the end of the wasm cranelift code for the given function.