Read OMP_TARGET_OFFLOAD and provide API to access ICV

Added settings code to read OMP_TARGET_OFFLOAD environment variable. Added
target-offload-var ICV as __kmp_target_offload, set via OMP_TARGET_OFFLOAD,
if available, otherwise defaulting to DEFAULT. Valid values for the ICV are
specified as enum values {0,1,2} for disabled, default, and mandatory. An
internal API access function __kmpc_get_target_offload is provided.

Patch by Terry Wilmarth

Differential Revision: https://reviews.llvm.org/D44577

llvm-svn: 328046