[nvptx] Set -misa=sm_35 by default

[nvptx] Set -misa=sm_35 by default

The nvptx-as assembler verifies the ptx code using ptxas, if there's any
in the PATH.

The default in the nvptx port for -misa=sm_xx is sm_30, but the ptxas of the
latest cuda release (11.1) no longer supports sm_30.

Consequently we cannot build gcc against that release (although we should
still be able to build without any cuda release).

Fix this by setting -misa=sm_35 by default.

Tested check-gcc on nvptx.

Tested libgomp on x86_64-linux with nvpx accelerator.

Both build again cuda 9.1.

gcc/ChangeLog:

2020-10-09  Tom de Vries  <tdevries@suse.de>

	PR target/97348
	* config/nvptx/nvptx.h (ASM_SPEC): Also pass -m to nvptx-as if
	default is used.
	* config/nvptx/nvptx.opt (misa): Init with PTX_ISA_SM35.