[nfc][libomptarget] Decrease coupling between files

Summary:
[nfc][libomptarget] Decrease coupling between files

debug.h used the symbol omptarget_device_environment so implicitly required
an include of omptarget-nvptx.h to compile. Similarly interface.h uses size_t.

Moving this declaration to a new header means cancel, critical can now build
without omptarget-nvptx.h. After this change, debug.h, cancel.cu, critical.cu
could move under a common source directory.

Reviewers: ABataev, jdoerfert, grokos

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D69473