[llvm] [openmp] [OpenMP] Fix runtimes default build (PR #149871)

[via llvm-commits]

jprotze wrote:

@madhur13490 can you provide information about the three stages (i.e., relevant cmake flags for all stages like llvm-projects/runtime, flags to customize openmp build/install paths)? 
Additionall, information about the location of omp.h before this patch and with this patch might be helpful to understand the problem.

https://github.com/llvm/llvm-project/pull/149871