LSan misses leaks due to miscalculating active stack regions.

While working on #15862 I notices that LSan would miss the memory leak unless the test was crafted carefully to avoid the stack region.

I believe the stack region is not been correctly calculated here:
https://github.com/emscripten-core/emscripten/blob/308c450312b9207eaf117fb9c7922591297dcb59/system/lib/compiler-rt/lib/lsan/lsan_common_emscripten.cpp#L147