[nvptx] Handle move from DF subreg to DF reg in nvptx_output_mov_insn

[nvptx] Handle move from DF subreg to DF reg in nvptx_output_mov_insn

When compiling test-case gcc.dg/atomic/c11-atomic-exec-1.c, we run into
these ptxas errors:
...
line 100; error: Rounding modifier required for instruction 'cvt'
line 105; error: Rounding modifier required for instruction 'cvt'
...

The problem is that this move:
...
//(insn 13 11 14 2
//      (set (reg:DF 28 [ _9 ])
//           (subreg:DF (reg:TI 22 [ _1 ]) 0)) 9 {*movdf_insn}
//       (nil))
                cvt.f64.u64     %r28, %r22$0;
...
is emitted as cvt.f64.u64, while it should be a mov.b64 instead.

Fix this by handling this case in nvptx_output_mov_insn.

Tested on nvptx.

gcc/ChangeLog:

	PR target/97158
	* config/nvptx/nvptx.c (nvptx_output_mov_insn): Handle move from
	DF subreg to DF reg.