Mark our PTX kernels as kernels, to stop them from being stripped (#8571)

* Mark our PTX kernels as kernels, to stop them from being stripped

Author: abadams

src/CodeGen_PTX_Dev.cpp

@@ -156,6 +156,8 @@ void CodeGen_PTX_Dev::add_kernel(Stmt stmt,
         }
     }
 
+    function->setCallingConv(llvm::CallingConv::PTX_Kernel);
+
     // Make the initial basic block
     entry_block = BasicBlock::Create(*context, "entry", function);
     builder->SetInsertPoint(entry_block);