[3.14] gh-128627: Use __builtin_wasm_test_function_pointer_signature for Emscripten trampoline (GH-137470) (#139039)

gh-128627: Use __builtin_wasm_test_function_pointer_signature for Emscripten trampoline (GH-137470)

With https://github.com/llvm/llvm-project/pull/150201 being merged, there is
now a better way to generate the Emscripten trampoline, instead of including
hand-generated binary WASM content. Requires Emscripten 4.0.12.
(cherry picked from commit 2629ee4eb0)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>

Python/emscripten_trampoline_inner.c

@@ -0,0 +1,38 @@
+// This file must be compiled with -mgc to enable the extra wasm-gc
+// instructions. It has to be compiled separately because not enough JS runtimes
+// support wasm-gc yet. If the JS runtime does not support wasm-gc (or has buggy
+// support like iOS), we will use the JS trampoline fallback.
+
+// We can't import Python.h here because it is compiled/linked with -nostdlib.
+// We don't need to know what's inside PyObject* anyways. We could just call it
+// void* everywhere. There are two reasons to do this:
+// 1. to improve readability
+// 2. eventually when we are comfortable requiring wasm-gc, we can merge this
+//    into emscripten_trampoline.c without worrying about it.
+typedef void PyObject;
+
+typedef PyObject* (*three_arg)(PyObject*, PyObject*, PyObject*);
+typedef PyObject* (*two_arg)(PyObject*, PyObject*);
+typedef PyObject* (*one_arg)(PyObject*);
+typedef PyObject* (*zero_arg)(void);
+
+#define TRY_RETURN_CALL(ty, args...) \
+  if (__builtin_wasm_test_function_pointer_signature((ty)func)) { \
+    return ((ty)func)(args); \
+  }
+
+__attribute__((export_name("trampoline_call"))) PyObject*
+trampoline_call(int* success,
+                void* func,
+                PyObject* self,
+                PyObject* args,
+                PyObject* kw)
+{
+  *success = 1;
+  TRY_RETURN_CALL(three_arg, self, args, kw);
+  TRY_RETURN_CALL(two_arg, self, args);
+  TRY_RETURN_CALL(one_arg, self);
+  TRY_RETURN_CALL(zero_arg);
+  *success = 0;
+  return 0;
+}