Win32: Expose wchar main routine only

Warned if both of `main` and `wmain` are exposed:
```
LINK : warning LNK4067: ambiguous entry point; selected 'mainCRTStartup'
```

main.c

@@ -43,6 +43,12 @@ int rb_wasm_rt_start(int (main)(int argc, char **argv), int argc, char **argv);
 #define rb_main(argc, argv) rb_wasm_rt_start(rb_main, argc, argv)
 #endif
 
+#ifdef _WIN32
+#define main(argc, argv) w32_main(argc, argv)
+static int main(int argc, char **argv);
+int wmain(void) {return main(0, NULL);}
+#endif
+
 int
 main(int argc, char **argv)
 {
@@ -56,7 +62,3 @@ main(int argc, char **argv)
     ruby_sysinit(&argc, &argv);
     return rb_main(argc, argv);
 }
-
-#ifdef _WIN32
-int wmain(void) {return main(0, NULL);}
-#endif

win32/winmain.c

@@ -1,10 +1,10 @@
 #include <windows.h>
 #include <stdio.h>
 
-extern int main(int, char**);
+extern int wmain(int, WCHAR**);
 
 int WINAPI
 WinMain(HINSTANCE current, HINSTANCE prev, LPSTR cmdline, int showcmd)
 {
-    return main(0, NULL);
+    return wmain(0, NULL);
 }