Move 'g++.dg/abi/nvptx-[...].C' -> 'g++.target/nvptx/abi-[...].C'

... instead of burying these nvptx-specific test cases within the generic ones.

	gcc/testsuite/
	* g++.dg/abi/nvptx-nrv1.C: Move...
	* g++.target/nvptx/abi-nrv1.C: ... here.
	* g++.dg/abi/nvptx-ptrmem1.C: Move...
	* g++.target/nvptx/abi-ptrmem1.C: ... here.

gcc/testsuite/g++.dg/abi/nvptx-nrv1.C → gcc/testsuite/g++.target/nvptx/abi-nrv1.C

-// { dg-do compile { target nvptx-*-* } }
+// { dg-do compile }
 // { dg-additional-options "-m64" }
 
 // Check NRV optimization doesn't change the PTX prototypes.

gcc/testsuite/g++.dg/abi/nvptx-ptrmem1.C → gcc/testsuite/g++.target/nvptx/abi-ptrmem1.C

-// { dg-do compile { target nvptx-*-* } }
+// { dg-do compile }
 // { dg-additional-options "-m64" }
 
 // ptr to member data is like sizeof.