Show the child process wait code instead of turning it into an error string

The exit code of the child process may not fit in 8 bits, it's more useful to provide the full child process status.

Fixes https://github.com/libsdl-org/SDL/issues/16188

src/process/posix/SDL_posixprocess.c

@@ -366,7 +366,7 @@ bool SDL_SYS_CreateProcessWithProperties(SDL_Process *process, SDL_PropertiesID
                 goto posix_spawn_fail_all;
             }
             if (status != 0) {
-                SDL_SetError("posix_spawn() failed: %s", strerror(status));
+                SDL_SetError("posix_spawn() failed: %d", status);
                 goto posix_spawn_fail_all;
             }
             break;