Skip IsRegularFileOrPipe() check on Emscripten

The sandbox guarantees that this will be true.

Fixes https://github.com/libsdl-org/SDL/issues/12108

(backported from from commit 1c7cc602863821ee62dac65d9e3c26fcc2710b3c)

src/file/SDL_rwops.c

@@ -537,7 +537,7 @@ static SDL_bool IsRegularFileOrPipe(FILE *f)
         !((st.st_mode & _S_IFMT) == _S_IFREG || (st.st_mode & _S_IFMT) == _S_IFIFO)) {
         return SDL_FALSE;
     }
-    #else
+    #elif !defined __EMSCRIPTEN__
     struct stat st;
     if (fstat(fileno(f), &st) < 0 || !(S_ISREG(st.st_mode) || S_ISFIFO(st.st_mode))) {
         return SDL_FALSE;