[Libomptarget][NFC] Rename device environment variable

This variable is used by the runtime. Before kernel launch we set it to
indicate several configuration options from the host. This patch renames
it to be more in-line with the rest of the named exported from the
runtime. This is better because this is the only symbol visible to the
host from the runtime, so it should have a reserved name.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D141960