nvptx: forward '-v' command-line option to assembler

nvptx: forward '-v' command-line option to assembler

For example, for offloading compilation with '-save-temps -v', before vs. after
word-diff then looks like:

    [...]
     [...]/build-gcc-offload-nvptx-none/gcc/as {+-v -v+} -o ./a.xnvptx-none.mkoffload.o ./a.xnvptx-none.mkoffload.s
    {+Verifying sm_30 code with sm_35 code generation.+}
    {+ ptxas -c -o /dev/null ./a.xnvptx-none.mkoffload.o --gpu-name sm_35 -O0+}
    [...]

(This depends on <https://github.com/MentorEmbedded/nvptx-tools/pull/37>
"Put '-v' verbose output onto stderr instead of stdout".)

	gcc/
	* config/nvptx/nvptx.h (ASM_SPEC): Define.