[nvptx] Workaround sub.u16 driver JIT bug

There's a nvidia driver JIT bug that mishandles this code (minimized from
builtin-arith-overflow-15.c):
...
int main (void) {
  signed char r;
  unsigned char y = (unsigned char) 0x80;
  if (__builtin_sub_overflow ((unsigned char)0, (unsigned char)y, &r))
    __builtin_abort ();
  return 0;
}
...
which at ptx level minimizes to:
...
  mov.u16 r22, 0x0080;
  st.local.u16 [frame_var],r22;
  ld.local.u16 r32,[frame_var];
  sub.u16 r33,0x0000,r32;
  cvt.u32.u16 r35,r33;
...
where we expect r35 == 0x0000ff80 but get instead 0xffffff80, and where using
nvptx-none-run -O0 fixes the problem.  [ See also
https://github.com/vries/nvidia-bugs/tree/master/builtin-arith-overflow-15 . ]

Try to workaround the bug by using sub.s16 instead of sub.u16.

Tested on nvptx.

gcc/ChangeLog:

2022-02-07  Tom de Vries  <tdevries@suse.de>

	PR target/97005
	* config/nvptx/nvptx.md (define_insn "sub<mode>3"): Workaround
	driver JIT bug by using sub.s16 instead of sub.u16.

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