Add script to automate WASM build

Automate WASM build with a new Python script. The script provides
several build profiles with configure flags for Emscripten flavors
and WASI. The script can detect and use Emscripten SDK and WASI SDK from
default locations or env vars.

``configure`` now detects Node arguments and creates HOSTRUNNER
arguments for Node 16. It also sets correct arguments for
``wasm64-emscripten``.

Author: tiran

Python/sysmodule.c

@@ -2789,14 +2789,18 @@ EM_JS(char *, _Py_emscripten_runtime, (void), {
     if (typeof navigator == 'object') {
         info = navigator.userAgent;
     } else if (typeof process == 'object') {
-        info = "Node.js ".concat(process.version)
+        info = "Node.js ".concat(process.version);
     } else {
-        info = "UNKNOWN"
+        info = "UNKNOWN";
     }
     var len = lengthBytesUTF8(info) + 1;
     var res = _malloc(len);
-    stringToUTF8(info, res, len);
+    if (res) stringToUTF8(info, res, len);
+#if __wasm64__
+    return BigInt(res);
+#else
     return res;
+#endif
 });
 
 static PyObject *

Tools/wasm/README.md

@@ -35,7 +35,13 @@ docker run --rm -ti -v $(pwd):/python-wasm/cpython -w /python-wasm/cpython quay.
 
 ### Compile a build Python interpreter
 
-From within the container, run the following commands:
+From within the container, run the following command:
+
+```shell
+./Tools/wasm/wasm_build.py build
+```
+
+The command is roughly equivalent to:
 
 ```shell
 mkdir -p builddir/build
@@ -45,13 +51,13 @@ make -j$(nproc)
 popd
 ```
 
-### Fetch and build additional emscripten ports
+### Cross compile to wasm32-emscripten for browser
 
 ```shell
-embuilder build zlib bzip2
+./Tools/wasm/wasm_build.py emscripten-browser
 ```
 
-### Cross compile to wasm32-emscripten for browser
+The command is roughly equivalent to:
 
 ```shell
 mkdir -p builddir/emscripten-browser
@@ -85,22 +91,29 @@ and header files with debug builds.
 ### Cross compile to wasm32-emscripten for node
 
 ```shell
-mkdir -p builddir/emscripten-node
-pushd builddir/emscripten-node
+./Tools/wasm/wasm_build.py emscripten-browser-dl
+```
+
+The command is roughly equivalent to:
+
+```shell
+mkdir -p builddir/emscripten-node-dl
+pushd builddir/emscripten-node-dl
 
 CONFIG_SITE=../../Tools/wasm/config.site-wasm32-emscripten \
   emconfigure ../../configure -C \
     --host=wasm32-unknown-emscripten \
     --build=$(../../config.guess) \
     --with-emscripten-target=node \
+    --enable-wasm-dynamic-linking \
     --with-build-python=$(pwd)/../build/python
 
 emmake make -j$(nproc)
 popd
 ```
 
 ```shell
-node --experimental-wasm-threads --experimental-wasm-bulk-memory --experimental-wasm-bigint builddir/emscripten-node/python.js
+node --experimental-wasm-threads --experimental-wasm-bulk-memory --experimental-wasm-bigint builddir/emscripten-node-dl/python.js
 ```
 
 (``--experimental-wasm-bigint`` is not needed with recent NodeJS versions)
@@ -234,6 +247,12 @@ The script ``wasi-env`` sets necessary compiler and linker flags as well as
 ``pkg-config`` overrides. The script assumes that WASI-SDK is installed in
 ``/opt/wasi-sdk`` or ``$WASI_SDK_PATH``.
 
+```shell
+./Tools/wasm/wasm_build.py wasi
+```
+
+The command is roughly equivalent to:
+
 ```shell
 mkdir -p builddir/wasi
 pushd builddir/wasi