Deal with W^X backport bustage in upcoming ESR release

https://hg.mozilla.org/releases/mozilla-esr45/rev/347c10e4d6d1 backports a security bug fix but is removing the nonWritableJitCode option we use for our W^X patch backport. It might be okay to add that part back as it seems the removal is unrelated. Jan de Mooij writes:

I also removed the nonWritableJitCode option as ESR45 predates W^X and nobody is going to enable that option there.

We should be sure about that though and make sure as well that our backported patches are still working and not causing unpredictable issues. If the risk is too high we need to back out the security improvements, alas.