[nvptx] Update default ptx isa to 6.3

[nvptx] Update default ptx isa to 6.3

With the following example, minimized from parallel-dims.c:
...
int
main (void)
{
  int vectors_max = -1;
  #pragma acc parallel num_gangs (1) num_workers (1) copy (vectors_max)
  {
    for (int i = 0; i < 2; i++)
      for (int j = 0; j < 2; j++)
        #pragma acc loop vector reduction (max: vectors_max)
        for (int k = 0; k < 32; k++)
          vectors_max = k;
  }

  if (vectors_max != 31)
    __builtin_abort ();

  return 0;
}
...
I run into (T400, driver version 470.94):
...
FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/parallel-dims.c \
  -DACC_DEVICE_TYPE_nvidia=1 -DACC_MEM_SHARED=0 -foffload=nvptx-none -O2 \
  execution test
...
The FAIL does not happen with GOMP_NVPTX_JIT=-O0.

The problem seems to be that the shfl insns for the vector reduction are not
executed uniformly by the warp.  Enforcing this by using shfl.sync fixes the
problem.

Fix this by setting the ptx isa to 6.3 by default, which allows the use of
shfl.sync.

Tested on x86_64 with nvptx accelerator.

gcc/ChangeLog:

2022-01-27  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.opt (mptx): Set to PTX_VERSION_6_3 by default.