graphics/opencv: when on POWER9 on powerpc64le, allow setting VSX3 as baseline

Also GCC is necessary because base LLVM 15 fails with:
UNREACHABLE executed at /usr/src/contrib/llvm-project/llvm/lib/Target/PowerPC/PPCISelLowering.cpp:11135!

LLVM 16 also crashes and it seems 17 has already fixed this issue.