Fix DECLARE_ASM_MODULE_EXPORTS=0 for mangled main function (#18895)

With `DECLARE_ASM_MODULE_EXPORTS` the list of wasm exports is statically
generated at build time.  This means that mangling in the compiler takes
care of converted `__main_argc_argv` to `main`.  e.g.  something like
the following is statically generated:

```
var main = Module['main] = asm['__main_argc_argv'];
```

However, with `DECLARE_ASM_MODULE_EXPORTS=0` the exports are dynamically
propagated from the wasm exports to the Module object, so we need to
apply the same demangling at runtime as we do at build time.

The build time version of this logic is here:
https://github.com/emscripten-core/emscripten/blob/c4693feae0e366cf7741e9e1725fda5539605d4d/tools/shared.py#L656-L657

Author: sbc100

src/library.js

@@ -3612,6 +3612,9 @@ mergeInto(LibraryManager.library, {
 
   $asmjsMangle: function(x) {
     var unmangledSymbols = {{{ buildStringArray(WASM_SYSTEM_EXPORTS) }}};
+    if (x == '__main_argc_argv') {
+      x = 'main';
+    }
     return x.indexOf('dynCall_') == 0 || unmangledSymbols.includes(x) ? x : '_' + x;
   },
 

test/declare_asm_module_exports.c

@@ -0,0 +1,17 @@
+#include <assert.h>
+#include <emscripten.h>
+#include <stdio.h>
+
+int EMSCRIPTEN_KEEPALIVE cFunction(void) { return 1; }
+
+EM_JS(int, jsFunction, (), {
+  return _cFunction();
+});
+
+// Intentional use of __main_argc_argv
+int main(int argc, char* argv[]) {
+  printf("cFunction: %d\n", cFunction());
+  printf("jsFunction: %d\n", jsFunction());
+  assert(jsFunction() == 1);
+  return 0;
+}

test/declare_asm_module_exports.cpp

@@ -5061,14 +5061,14 @@ def test_closure_in_web_only_target_environment_webgl(self):
   def test_no_declare_asm_module_exports_asmjs(self, args):
     # TODO(sbc): Fix closure warnings with MODULARIZE + WASM=0
     self.ldflags.append('-Wno-error=closure')
-    self.btest(test_file('declare_asm_module_exports.cpp'), '1', args=['-sDECLARE_ASM_MODULE_EXPORTS=0', '-sENVIRONMENT=web', '-O3', '--closure=1', '-sWASM=0'] + args)
+    self.btest_exit(test_file('declare_asm_module_exports.c'), args=['-sDECLARE_ASM_MODULE_EXPORTS=0', '-sENVIRONMENT=web', '-O3', '--closure=1', '-sWASM=0'] + args)
 
   @parameterized({
     '': (1,),
     '2': (2,),
   })
   def test_no_declare_asm_module_exports_wasm_minimal_runtime(self, mode):
-    self.btest(test_file('declare_asm_module_exports.cpp'), '1', args=['-sDECLARE_ASM_MODULE_EXPORTS=0', '-sENVIRONMENT=web', '-O3', '--closure=1', f'-sMINIMAL_RUNTIME={mode}'])
+    self.btest_exit(test_file('declare_asm_module_exports.c'), args=['-sDECLARE_ASM_MODULE_EXPORTS=0', '-sENVIRONMENT=web', '-O3', '--closure=1', f'-sMINIMAL_RUNTIME={mode}'])
 
   # Tests that the different code paths in src/shell_minimal_runtime.html all work ok.
   def test_minimal_runtime_loader_shell(self):

test/test_browser.py

@@ -8792,7 +8792,7 @@ def test_no_declare_asm_module_exports(self):
     self.set_setting('DECLARE_ASM_MODULE_EXPORTS', 0)
     self.set_setting('WASM_ASYNC_COMPILATION', 0)
     self.maybe_closure()
-    self.do_runf(test_file('declare_asm_module_exports.cpp'), 'jsFunction: 1')
+    self.do_runf(test_file('declare_asm_module_exports.c'), 'jsFunction: 1')
     js = read_file('declare_asm_module_exports.js')
     occurances = js.count('cFunction')
     if self.is_optimizing() and '-g' not in self.emcc_args:
@@ -8814,7 +8814,7 @@ def test_minimal_runtime_no_declare_asm_module_exports(self):
     self.maybe_closure()
     self.set_setting('MINIMAL_RUNTIME')
     self.emcc_args += ['--pre-js', test_file('minimal_runtime_exit_handling.js')]
-    self.do_runf(test_file('declare_asm_module_exports.cpp'), 'jsFunction: 1')
+    self.do_runf(test_file('declare_asm_module_exports.c'), 'jsFunction: 1')
 
   # Tests that -sMINIMAL_RUNTIME works well in different build modes
   @no_wasmfs('https://github.com/emscripten-core/emscripten/issues/16816')