Add a stub __cxa_free_exception to cxa_noexception.cpp

[dschuff]

In some cases cxa_free_exception can be called by compiler-generated code, which may cause linkage failure in emscripten's default mode which allows throwing but not catching. This PR adds a "stub" __cxa_free_exception to go along with __cxa_allocate_exception.

[dschuff]

This does fix the LLVM testsuite failure seen in #16627
It's not completely clear to me whether alloc/free should actually do something in this case or not. IIUC in the default mode, throws become aborts, so maybe actually allocating and freeing is unnecessary. But this implementation mirrors the allocation.

[sbc100]

We should add a test but I guess we can rely on the llvm testsuite for now.

[dschuff]

Yeah I think the llvm testsuite is ok coverage for now. I did verify it locally.