Switching from building without debug-mozjs to building with fails to link

[bzbarsky]

STEPS TO REPRODUCE:

1. ./mach build --dev
2. ./mach build --dev --debug-mozjs

EXPECTED RESULTS: build works

ACTUAL RESULTS: Link failures like this:

Undefined symbols for architecture x86_64:
"js::oom::maxAllocations", referenced from:
js::oom::IsSimulatedOOMAllocation() in libjs-dd291aac52e4d65e.rlib(jsglue.cpp.o)

[nox]

Caused by servo/mozjs#97.

[nox]

This is still true.

[nox]

There are no link errors anymore but it still takes forever to (re)build.

[nox]

Actually, nothing more can be done here, it does reuse artefacts from one debug-mozjs build to the next one and it doesn't fail to link.