filesystem: SDL_GetBasePath() now follows the SDL_GetStringRule.

It also now caches at the higher level, so the platform-specific bits don't
change their interface much.

A little code hygiene work was applied to some of the platform bits on top of
this.

Reference Issue #10229.

src/filesystem/emscripten/SDL_sysfilesystem.c

@@ -29,10 +29,9 @@
 
 #include <emscripten/emscripten.h>
 
-char *SDL_GetBasePath(void)
+char *SDL_SYS_GetBasePath(void)
 {
-    char *retval = "/";
-    return SDL_strdup(retval);
+    return SDL_strdup("/");
 }
 
 char *SDL_GetPrefPath(const char *org, const char *app)