Move WasmFS C++ link workaround from test code to emcc.py

[kripken]

This way WasmFS can be used outside the test suite as well.

Followup to #15147

[kripken]

@sbc100 wasm2.test_pthread_nested_work_queue does look like a new flake - I saw it on this PR before the last change, and now it didn't show up. But it did appear twice today in two different places, hm...

[sbc100]

If its flaky enough maybe we could bisect it?

[kripken]

Hmm, last commits look ok actually... 😄 who knows. Actually, perhaps this was a temporary breakage on tot due to some LLVM change that they reverted, perhaps.