games/oblige: fix build on powerpc*

gui/sys_endian.h:66:58: error: unsupported inline asm: input with type 'int' matching output with type 'u16_t' (aka 'unsigned short')
  __asm__("rlwimi %0,%2,8,16,23" : "=&r" (result) : "0" (x >> 8), "r" (x));
                                          ~~~~~~         ^~~~~~