Restore emscripten to Tier 3.

Author: freakboy3742

peps/pep-0011.rst

@@ -128,6 +128,7 @@ powerpc64le-unknown-linux-gnu    glibc, clang                Victor Stinner
 
                                  glibc, gcc                  Victor Stinner
 s390x-unknown-linux-gnu          glibc, gcc                  Victor Stinner
+wasm32-unknown-emscripten        emcc                        Russell Keith-Magee
 x86_64-linux-android                                         Russell Keith-Magee, Petr Viktorin
 x86_64-unknown-freebsd           BSD libc, clang             Victor Stinner
 ================================ =========================== ========