astro/opencpn: fix build on powerpc64*

Use GCC unconditionally on powerpc64*:
In file included from /usr/lib/clang/11.0.1/include/xmmintrin.h:13:
/usr/lib/clang/11.0.1/include/mmintrin.h:33:5: error: use of undeclared identifier 'builtin_ia32_emms'; did you mean 'builtin_isless'?

__builtin_ia32_emms();