[3.14] gh-127146: Emscripten: Make os.umask() actually work (GH-136706) (#136711)

Provide a stub implementation of umask that is enough to get some tests passing.
More work is needed upstream in Emscripten to make all umask tests to pass.
(cherry picked from commit 12e52cad718723636a96042f9399634392285c44)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>