The only use is with %#x, where we'll automatically get 0x prefix.
Advantage over 0x%x can be seen with padding.

A simple test:

  printf(".%#08x.\n", 0);
  printf(".%#8x.\n",  0);
  printf(".%#-8x.\n", 0);

  printf(".%#08x.\n", 1);
  printf(".%#8x.\n",  1);
  printf(".%#-8x.\n", 1);

  printf(".%#08x.\n", 0x123456);
  printf(".%#8x.\n",  0x123456);
  printf(".%#-8x.\n", 0x123456);

  printf(".%#02x.\n", 0);
  printf(".%#2x.\n",  0);
  printf(".%#-2x.\n", 0);

  printf(".%#02x.\n", 1);
  printf(".%#2x.\n",  1);
  printf(".%#-2x.\n", 1);

looks just like glibc:

  .00000000.
  .       0.
  .0       .
  .0x000001.
  .     0x1.
  .0x1     .
  .0x123456.
  .0x123456.
  .0x123456.
  .00.
  . 0.
  .0 .
  .0x1.
  .0x1.
  .0x1.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>