[3.14] gh-145219: Add Emscripten cross-build and clean configurability (GH-145581) (#145654)

Modifies the Emscripten build script to allow for custom cross-build directory
names, and to only clean Emscripten-specific paths (optionally including the
build python).
(cherry picked from commit 015613384fea7a00bb2077760e325e5baab6814b)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>
Co-authored-by: Russell Keith-Magee <russell@keith-magee.com>