[OpenMP] Unify "exec_mode" query code and default to SPMD

Defaulting to Generic mode doesn't make much sense as the kernel needs
to be prepared for it. SPMD mode is the "native" execution, e.g., for
"bare" kernels. It also is the execution method for constructors and
destructors (as we might otherwise throw an extra warp onto them).

Differential Revision: https://reviews.llvm.org/D140718