[OpenMP] Make the new device runtime the default

This patch changes the `-fopenmp-target-new-runtime` option which controls if
the new or old device runtime is used to be true by default.  Disabling this to
use the old runtime now requires using `-fno-openmp-target-new-runtime`.

Reviewed By: JonChesterfield, tianshilei1992, gregrodgers, ronlieb

Differential Revision: https://reviews.llvm.org/D114890