nvptx: Add -misa=sm_70

nvptx: Add -misa=sm_70

Add -misa=sm_70, and use it to specify the misa value in test-case
gcc.target/nvptx/atomic-store-2.c.

Tested on nvptx.

gcc/ChangeLog:

	* config/nvptx/nvptx-c.cc (nvptx_cpu_cpp_builtins): Handle SM70.
	* config/nvptx/nvptx.cc (first_ptx_version_supporting_sm):
	Likewise.
	* config/nvptx/nvptx.opt (misa): Add sm_70 alias PTX_ISA_SM70.

gcc/testsuite/ChangeLog:

2022-02-22  Tom de Vries  <tdevries@suse.de>

	* gcc.target/nvptx/atomic-store-2.c: Use -misa=sm_70.
	* gcc.target/nvptx/uniform-simt-3.c: Same.

Co-Authored-By: Tom de Vries <tdevries@suse.de>