Add test for NVPTX intrinsics

[thewilsonator]

cc @JohanEngelen

#2556

Whats the best way to do this for all targets that LDC supports?

[JohanEngelen]

This test should include actual usage of an intrinsic (adding dcompute_ as prefix to the filename would be nice too).
The idea is not to test the existence of the intrinsc file, but its correctness (for example, the test should catch what Kai forgot to add orginally: "The generated file will have to be marked @compute(CompileFor.deviceOnly), with the corresponding import of ldc.dcompute.").

Please make it part of #2556 (or add those changes to this PR).