[CMake][OpenMP] Customize default offloading arch

For the shuffle instructions in reductions we need at least sm_30
but the user may want to customize the default architecture.

Differential Revision: https://reviews.llvm.org/D38883

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315996 91177308-0d34-0410-b5e6-96231b3b80d8