[Libomptarget] Correctly default to Generic if exec_mode is not present

Currently, the runtime returns an error when the `exec_mode` global is
not present. The expected behvaiour is that the region will default to
Generic. This prevents global constructors from being called because
they do not contain execution mode globals.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D108255