Add Emscripten libmpdec build step

Goes together with:
python/cpython#136997

Author: hoodmane

master/custom/factories.py

@@ -1365,10 +1365,15 @@ def setup(self, **kwargs):
                 env=compile_environ,
             ),
             Compile(
-                name="Compile host libFFI",
+                name="Compile host libffi",
                 command=["python3", "Tools/wasm/emscripten", "make-libffi"],
                 env=compile_environ,
             ),
+            Compile(
+                name="Compile host libmpdecimal",
+                command=["python3", "Tools/wasm/emscripten", "make-mpdec"],
+                env=compile_environ,
+            ),
             Configure(
                 name="Configure host Python",
                 command=["python3", "Tools/wasm/emscripten", "configure-host"],