[nvptx] Basic -misa support for nvptx

[nvptx] Basic -misa support for nvptx

	gcc/
	* config/nvptx/nvptx-opts.h: New file.
	* config/nvptx/nvptx.c (nvptx_file_start): Print the correct .target.
	* config/nvptx/nvptx.h: Include "nvptx-opts.h".
	(ASM_SPEC): Define.
	(TARGET_SM35): New macro.
	* config/nvptx/nvptx.md (atomic_fetch_<logic><mode>): Enable with the
	correct predicate.
	* config/nvptx/nvptx.opt (ptx_isa, sm_30, sm_35): New enum and its
	values.
	(misa=): New option.
	* doc/invoke.texi (Nvidia PTX Options): Document -misa.

	gcc/testsuite/
	* gcc.target/nvptx/atomic_fetch-1.c: New test.
	* gcc.target/nvptx/atomic_fetch-1.c: New test.

Co-Authored-By: Bernd Schmidt <bernds_cb1@t-online.de>
From-SVN: r264133