[CUDA] Disable PGO and coverage instrumentation in NVPTX.

NVPTX does not have runtime support necessary for profiling to work
and even call arc collection is prohibitively expensive. Furthermore,
there's no easy way to collect the samples. NVPTX also does not
support global constructors that clang generates if sample/arc collection
is enabled.

Differential Revision: https://reviews.llvm.org/D42452

llvm-svn: 323345