Add 'g++.target/nvptx/nvptx.exp' for nvptx-specific C++ test cases

Like 'gcc.target/nvptx/nvptx.exp' is modeled after 'gcc.dg/dg.exp', this new
'g++.target/nvptx/nvptx.exp' is modeled after 'g++.dg/dg.exp'.

	gcc/testsuite/
	* g++.target/nvptx/nvptx.exp: New.