cmd/cgo: use full prototype for main in C code

Avoids pedantic errors from modern C compilers.

Fixes #70769

Change-Id: Ibe0d9960e998eb0346b97d7bd69eb7de0d0e6c60
Reviewed-on: https://go-review.googlesource.com/c/go/+/635095
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/cgo/doc.go

@@ -796,7 +796,7 @@ Instead, the build process generates an object file using dynamic
 linkage to the desired libraries. The main function is provided by
 _cgo_main.c:
 
-	int main() { return 0; }
+	int main(int argc, char **argv) { return 0; }
 	void crosscall2(void(*fn)(void*), void *a, int c, uintptr_t ctxt) { }
 	uintptr_t _cgo_wait_runtime_init_done(void) { return 0; }
 	void _cgo_release_context(uintptr_t ctxt) { }

src/cmd/cgo/out.go

@@ -59,7 +59,7 @@ func (p *Package) writeDefs() {
 
 	// Write C main file for using gcc to resolve imports.
 	fmt.Fprintf(fm, "#include <stddef.h>\n") // For size_t below.
-	fmt.Fprintf(fm, "int main() { return 0; }\n")
+	fmt.Fprintf(fm, "int main(int argc __attribute__((unused)), char **argv __attribute__((unused))) { return 0; }\n")
 	if *importRuntimeCgo {
 		fmt.Fprintf(fm, "void crosscall2(void(*fn)(void*) __attribute__((unused)), void *a __attribute__((unused)), int c __attribute__((unused)), size_t ctxt __attribute__((unused))) { }\n")
 		fmt.Fprintf(fm, "size_t _cgo_wait_runtime_init_done(void) { return 0; }\n")