Remove ".dll" from DYNAMICLIB_ENDINGS.

Remove ".dll" from DYNAMICLIB_ENDINGS, since on Windows one never links to a .dll file directly (e.g. via a command line -lFile -> libFile.dll) either, but instead separate import libraries with suffix .lib are used. Note that for crosscompiling, it is _not_ recommended to generate Emscripten static bitcode libraries with suffix .lib, but instead generating files with suffixes .bc (or .a) is better form.