[OpenMP] Add flag for setting debug in the offloading device

This patch introduces the flags `-fopenmp-target-debug` and
`-fopenmp-target-debug=` to set the value of a global in the device.
This will be used to enable or disable debugging features statically in
the device runtime library.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D109544