VecGeom Target Architecture Config with VecCore CMake Modules
This does minimal changes only to support -DARCH=(sse,sse4.2,avx,knc,knl,native,etc)
to configure the target architecture for VecGeom and VecCore together.
KNL build is also supported for gcc-5.3.0 and above if using the scalar backend (gcc-5.3.0 has no support for AVX512 intrinsics yet).
No changes are made to CUDA options for VecGeom in this branch, so I expect the builds to pass.