This increases the maximum available memory from 2GB
(Emscritpten default) to 4GB, which is the 32-bit wasm
max.
Add QT_WASM_MAXIMUM_MEMORY qmake/cmake option for overriding.
Pick-to: 6.7
Change-Id: I6257fc919a749412c4ba1e0f939996c6057ce314
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>