[CUDA][Windows] Restrict long double device functions declarations to Windows

As agreed in D60220, make long double declarations unobservable on non-windows platforms.

[Testing]
{Windows 10, Ubuntu 16.04.5}/{Visual C++ 2017 15.9.11 & 2019 16.0.1, gcc+ 5.4.0}/CUDA {8.0, 9.0, 9.1, 9.2, 10.0, 10.1}

Reviewed by: Artem Belevich

Differential Revision: https://reviews.llvm.org/D60818

llvm-svn: 358654