nvptx: Enhance '-mptx=[...]' test cases

nvptx: Enhance '-mptx=[...]' test cases

This expands upon the test cases added in
commit a2eacdbd4c4a698b3b6f27ef5e1f8dd3d836b2e5
"[nvptx] Add __PTX_ISA_VERSION_{MAJOR,MINOR}__".

	gcc/testsuite/
	* gcc.target/nvptx/ptx31.c: Remove; expanded into...
	* gcc.target/nvptx/mptx=3.1.c: ... this.
	* gcc.target/nvptx/ptx60.c: Remove; expanded into...
	* gcc.target/nvptx/mptx=6.0.c: ... this.
	* gcc.target/nvptx/ptx63.c: Remove; expanded into...
	* gcc.target/nvptx/mptx=6.3.c: ... this.
	* gcc.target/nvptx/ptx70.c: Remove; expanded into...
	* gcc.target/nvptx/mptx=7.0.c: ... this.
	* gcc.target/nvptx/mptx=_.c: New.