Make <nv/target> usable in C and C++98 compilations

[dkolsen-pgi]

The features of <nv/target> are only available in C++11 CUDA mode. But some headers that use <nv/target> can be compiled, with limited functionality, as C or as C++98. <nv/target> needs to do the same.

This change allows <nv/target> to be included successfully when compiling as C or as C++98. In this situation, "if target" won't
work because the namespace nv::target won't exist, and the compatibility macros will assume host-only.

[wmaxey]

We have no configured C++03 testing or compiler configurations, were you able to run this on NVC++ and others yourself?

[dkolsen-pgi]

I manually tested the change by compiling this program every way I could, with and without CUDA turned on, with nvcc, nvc++, and gcc, and as C, C++98, C++11, and C++14.

#include <stdio.h>
#include <nv/target>
int main() {
  NV_IF_TARGET(NV_IS_HOST, (
    printf("Passed\n");
  ), (
    printf("FAILED: This block should never be run.\n");
  ));
}

[wmaxey]

Rebased and launched virtual, I will submit when clean.

http://scdvs.nvidia.com/Build_Results?virtualId=1000072758&which_page=current_build