[CUDA] Mark CUDA as a no-errno platform.

Summary:
CUDA doesn't support errno at all, so this is the right thing -- or at
least, in the right direction.

But also, this unbreaks the CUDA test-suite math tests [0] after D39481.
__cuda_cmath.h forwards nexttoward() to __builtin_nexttoward, which,
after that patch, was lowering to a libcall that doesn't exist in NVPTX.

[0] http://lab.llvm.org:8011/builders/clang-cuda-build/builds/14999

Reviewers: tra

Subscribers: sanjoy, cfe-commits

Differential Revision: https://reviews.llvm.org/D39586

llvm-svn: 317297