Revert "Mark functions that take the address of a __global__ function as host…"

[jaredhoberock]

Reverts #835 due to bad interaction with nvcc 7.5.

[3gx]

It can be clang-specific like this:

instead of

  __host__ __device__
static global_function_pointer_t global_function_pointer() {

try this, assuming clang will never be able to do device-side launches:

#define THRUST_CLANG_RUNTIME_FUNCTION __host__ __device__
#if defined(__clang__) && defined(__CUDA__)
#define THRUST_CLANG_RUNTIME_FUNCTION ___host__
#endif 

THRSUT_CLANG_RUNTIME_FUNCTION
static global_function_pointer_t global_function_pointer() {