Forcefully disable warnings in GCC for <nv/target>

[wmaxey]

Currently GCC 6 and down produces this warning when compile with -Wpedantic

<source>:594:1: warning: ISO C++11 requires at least one argument for the "..." in a variadic macro
     )

While it's possible to ignore -Wpedantic, I believe it would be more reliable to silence warnings by marking the details of <nv/target> as system_headers.

[dkolsen-pgi]

Clang seems to recognize and honor #pragma GCC system_header. (At least Clang 12 on Linux does.) Is the !defined(__clang__) necessary?

[wmaxey]

I would be willing to remove the condition, but it doesn't seem necessary as I haven't encountered warnings on Clang over this.

[dkolsen-pgi]

I am fine with the change as it is, even though I think it is cleaner without the Clang check. (NVC++ also defines __GNUC__, but you didn't exclude that compiler. I checked that NVC++ also recognizes and honors that pragma.)