graphics/osgearth: fix build on powerpc64 head

Use GCC for its SSE - AltiVec translation.

In file included from /wrkdirs/usr/ports/graphics/osgearth/work/osgearth-osgearth-2.10.1/src/osgEarthDrivers/fastdxt/intrinsic.cpp:42:
In file included from /usr/lib/clang/11.0.0/include/emmintrin.h:13:
In file included from /usr/lib/clang/11.0.0/include/xmmintrin.h:13:
/usr/lib/clang/11.0.0/include/mmintrin.h:33:5: error: use of undeclared identifier 'builtin_ia32_emms'; did you mean 'builtin_isless'?

__builtin_ia32_emms();