games/openclonk: fix build on powerpc64le

Use GCC for its SSE -> AltiVec translation:
/usr/lib/clang/11.0.1/include/mmintrin.h:50:12: error: invalid conversion between vector type '__m64' (vector of 1 'long long' value) and integer type 'int' of different size

return (__m64)__builtin_ia32_vec_init_v2si(__i, 0);