[CUDA][Windows] Partial fix for bug #38811 (Step 1 of 3)

Partial fix for the clang Bug https://bugs.llvm.org/show_bug.cgi?id=38811 "Clang fails to compile with CUDA-9.x on Windows".

Adding defined(_WIN64) check along with existing #if defined(__LP64__) eliminates the below clang (64-bit) compilation error on Windows.

C:/GIT/LLVM/trunk/llvm-64-release-vs2017/dist/lib/clang/9.0.0\include\__clang_cuda_device_functions.h(1609,45): error GEF7559A7: no matching function for call to 'roundf'
 __DEVICE__ long lroundf(float __a) { return roundf(__a); }

Reviewed by: Artem Belevich

Differential Revision: http://reviews.llvm.org/D59361

llvm-svn: 356255