nvptx: Clarify that our baseline is PTX ISA Version 3.1

nvptx: Clarify that our baseline is PTX ISA Version 3.1

Added in commit decde11183bdccc46587d6614b75f3d56a2f2e4a
"[nvptx] Choose -mptx default based on -misa", 'PTX_VERSION_3_0' was added for
'first_ptx_version_supporting_sm' to return it for 'PTX_ISA_SM30' (as
documented by Nvidia).  It's however then immediately overridden to 3.1, which
in GCC/nvptx "has been the smallest version historically", and also '-mptx=3.0'
isn't exposed to the user.  As we also elsewhere (machine description etc.)
assume that our baseline is PTX ISA Version 3.1, there's no real value added in
maintaining 'PTX_VERSION_3_0' for purposes of 'first_ptx_version_supporting_sm'
only.

No change in behavior intended.

	gcc/
	* config/nvptx/nvptx-opts.h (enum ptx_version): Remove
	'PTX_VERSION_3_0'.
	* config/nvptx/nvptx.cc (first_ptx_version_supporting_sm)
	(default_ptx_version_option, ptx_version_to_string)
	(ptx_version_to_number): Adjust.
	* config/nvptx/nvptx.h: Comment.