[OPENMP] Implementation of omp_get_default_device and omp_set_default_device

Implementation of missing OpenMP 4.0 API functions omp_get_default_device and omp_set_default_device.
Also, added support for the environment variable OMP_DEFAULT_DEVICE.

Differential Revision: https://reviews.llvm.org/D23587

llvm-svn: 281065