Monkey-patch platform.machine for pip in pyodide venv (pyodide#4412)

Without this fix, `pip install sqlalchemy>=2` fails trying to install greenlet.

pyodide-build/pyodide_build/out_of_tree/venv.py

@@ -123,6 +123,7 @@ def get_executable():
         sys.platform = sys_platform
         sys.implementation._multiarch = multiarch
         platform.system = lambda: sys_platform
+        platform.machine = lambda: "wasm32"
         os.environ["_PYTHON_HOST_PLATFORM"] = host_platform
         os.environ["_PYTHON_SYSCONFIGDATA_NAME"] = f'_sysconfigdata_{{sys.abiflags}}_{{sys.platform}}_{{sys.implementation._multiarch}}'
         sys.path.append("{sysconfigdata_dir}")