[libomptarget] Fix devicertl build

[libomptarget] Fix devicertl build

The target specific functions in target_interface are extern C, but the
implementations for nvptx were mostly C++ mangling. That worked out as
a quirk of DEVICE macro expanding to nothing, except for shuffle.h which
only forward declared the functions with C++ linkage.

Also implements GetWarpSize, as used by shuffle, and includes target_interface
in nvptx target_impl.cu to help catch future divergence between interface and
implementation.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D98651