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

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

This expands upon the test cases added in
commit 4706670cd3b06bb024da0683776bf86c79d55940
"[nvptx, testsuite] Add gcc.target/nvptx/sm*.c".

	gcc/testsuite/
	* gcc.target/nvptx/sm30.c: Remove; expanded into...
	* gcc.target/nvptx/march=sm_30.c: ... this.
	* gcc.target/nvptx/sm35.c: Remove; expanded into...
	* gcc.target/nvptx/march=sm_35.c: ... this.
	* gcc.target/nvptx/sm53.c: Remove; expanded into...
	* gcc.target/nvptx/march=sm_53.c: ... this.
	* gcc.target/nvptx/sm70.c: Remove; expanded into...
	* gcc.target/nvptx/march=sm_70.c: ... this.
	* gcc.target/nvptx/sm75.c: Remove; expanded into...
	* gcc.target/nvptx/march=sm_75.c: ... this.
	* gcc.target/nvptx/sm80.c: Remove; expanded into...
	* gcc.target/nvptx/march=sm_80.c: ... this.
	* gcc.target/nvptx/march.c: Remove.