emscripten: Move some `EM_ASM` blocks to `MAIN_THREAD_EM_ASM`.

These blocks reference the `navigator` global, which is not available in
background threads.

(cherry picked from commit c728e2f044ef53dba0b0981e54ee68c7348b9f1b)

src/joystick/emscripten/SDL_sysjoystick.c

@@ -449,7 +449,7 @@ static bool EMSCRIPTEN_JoystickOpen(SDL_Joystick *joystick, int device_index)
     joystick->nbuttons = item->nbuttons;
     joystick->naxes = item->naxes;
 
-    rumble_available = EM_ASM_INT({
+    rumble_available = MAIN_THREAD_EM_ASM_INT({
         let gamepads = navigator['getGamepads']();
         if (!gamepads) {
             return 0;
@@ -570,7 +570,7 @@ static bool EMSCRIPTEN_JoystickRumble(SDL_Joystick *joystick, Uint16 low_frequen
     SDL_joylist_item *item = (SDL_joylist_item *)joystick->hwdata;
 
     // clang-format off
-    bool result = EM_ASM_INT({
+    bool result = MAIN_THREAD_EM_ASM_INT({
         let gamepads = navigator['getGamepads']();
         if (!gamepads) {
             return 0;

src/locale/emscripten/SDL_syslocale.c

@@ -27,7 +27,7 @@
 bool SDL_SYS_GetPreferredLocales(char *buf, size_t buflen)
 {
     /* *INDENT-OFF* */ // clang-format off
-    EM_ASM({
+    MAIN_THREAD_EM_ASM({
         var buf = $0;
         var buflen = $1;
         var list = undefined;