[nvptx] Add -mptx=7.0

Add support for ptx isa version 7.0, required for the addition of -misa=sm_75
and -misa=sm_80.

Tested by setting the default ptx isa version to 7.0, and doing a build and
libgomp test run.

gcc/ChangeLog:

	* config/nvptx/nvptx-opts.h (enum ptx_version): Add PTX_VERSION_7_0.
	* config/nvptx/nvptx.c (nvptx_file_start): Handle TARGET_PTX_7_0.
	* config/nvptx/nvptx.h (TARGET_PTX_7_0): New macro.
	* config/nvptx/nvptx.opt (ptx_version): Add 7.0.