[libomptarget-nvptx] Enable asserts in bclib

If the user requested LIBOMPTARGET_NVPTX_DEBUG, include asserts in
the bitcode library. Everything else will have very unpleasent
effects because asserts will appear when falling back to the static
library libomptarget-nvptx.a.

Differential Revision: https://reviews.llvm.org/D52701

llvm-svn: 343477