[nvptx] Choose -mptx default based on -misa

[nvptx] Choose -mptx default based on -misa

While testing with driver version 390.147 I ran into the problem that it
doesn't support ptx isa version 6.3 (the new default), only 6.1.

Furthermore, using the -mptx option is a bit user-unfriendly.

Say we want to compile for sm_80.  We can use -misa=sm_80 to specify that, but
then run into errors because the default ptx version is 6.3, which doesn't
support sm_80 yet.

Address both these issues by:
- picking a default -mptx based on the active -misa, and
- ensuring that the default -mptx is at least 6.0 (instead
  of 6.3).

Also add an error in case of incompatible options like
"-misa=sm_80 -mptx=6.3":
...
cc1: error: PTX version (-mptx) needs to be at least 7.0 to support \
  selected -misa (sm_80)
...

Tested on x86_64-linux with nvptx accelerator.

gcc/ChangeLog:

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

	PR target/104283
	* config/nvptx/nvptx-opts.h (enum ptx_version): Add PTX_VERSION_3_0
	and PTX_VERSION_4_2.
	* config/nvptx/nvptx.cc (first_ptx_version_supporting_sm)
	(default_ptx_version_option, ptx_version_to_string)
	(sm_version_to_string, handle_ptx_version_option): New function.
	(nvptx_option_override): Call handle_ptx_version_option.
	(nvptx_file_start): Use ptx_version_to_string and sm_version_to_string.
	* config/nvptx/nvptx.md (define_insn "nvptx_shuffle<mode>")
	(define_insn "nvptx_vote_ballot"): Use TARGET_PTX_6_0.
	* config/nvptx/nvptx.opt (mptx): Remove 'Init'.