[nvptx] Unbreak build, add PTX_ISA_SM70

With the commit "[nvptx] Choose -mptx default based on -misa" I introduced a
use of PTX_ISA_SM70, without adding it first.

Add it, as well as the corresponding TARGET_SM70.

Build for x86_64 with nvptx accelerator.

gcc/ChangeLog:

2022-02-08  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx-opts.h (enum ptx_isa): Add PTX_ISA_SM70.
	* config/nvptx/nvptx.h (TARGET_SM70): Define.