Add default feature for JIT

[jschwe]

This allows optionally disabling JIT by removing the feature. This is necessary on some variants of OpenHarmony, when JIT is forbidden by a system policy.

My idea is that on the servo side, I would disable default-features for mozjs when target_env = "ohos", since the dependency on mozjs is in the script crate. One could then add a direct dependency in servoshell on mozjs (again only for OpenHarmony), so that we can expose a top-level feature js-jit to the user running ./mach, since they are the only one that know if their distribution of openharmony allows jit or not.

Edit:
on the servo side I previously tried setting all the following prefs to false, which still caused JIT related crashes:

  "js.asmjs.enabled": false,
  "js.baseline_interpreter.enabled": false,
  "js.baseline_jit.enabled": false,
  "js.baseline_jit.unsafe_eager_compilation.enabled": false,
  "js.ion.enabled": false,
  "js.ion.offthread_compilation.enabled": false,
  "js.ion.unsafe_eager_compilation.enabled": false,

[sagudev]

I wonder if android has same problem if we forbid JIT by system?

[jschwe]

Is it possible to forbid JIT on android? If so, I'd assume we would hit the same issue.

[sagudev]

Is it possible to forbid JIT on android? If so, I'd assume we would hit the same issue.

I imagine it should be possible using SELinux policies.

BTW I think you also need to disable wasm JIT (wasm_baselinejit, wasm_ionjit) although I am not sure we expose those prefs.

[sagudev]

Or maybe we should call https://doc.servo.org/mozjs/jsapi/fn.DisableJitBackend.html before SM init in servo to fully disable JIT. Some doc in mozilla: https://searchfox.org/mozilla-central/source/js/public/Initialization.h#172

This is used on IOS which does not allow JIT: https://searchfox.org/mozilla-central/source/modules/libpref/init/StaticPrefList.yaml#8689

[sagudev]

I am going to close this given the main usecase was resolved by servo/servo#34231.