C - Fix padding numbering when structs are omitted

[stefanlippuner]

With the change from #55, I've introduced/triggered a new issue :| If padding is inserted at the top-level and within a block omitted prefix, the same name is given to the padding variable resulting in compilation errors, e.g.

#ifndef __ASSEMBLER__
struct blkprefix3 {
  /* [0x0]: BLOCK (comment missing) */
  /* [0x0]: REG (rw) (comment missing) */
  uint32_t r1;

  /* padding to: 8 Bytes */
  uint32_t __padding_0[1];

  /* [0x8]: REG (rw) (comment missing) */
  uint64_t r2;

  /* [0x10]: BLOCK (comment missing) */
  /* [0x0]: REG (rw) (comment missing) */
  uint32_t r3;

  /* padding to: 8 Bytes */
  uint32_t __padding_0[1];

  /* [0x8]: REG (rw) (comment missing) */
  uint64_t r4;

  /* [0x20]: BLOCK (comment missing) */
  struct b2 {
    /* [0x0]: REG (rw) (comment missing) */
    uint32_t r1;

    /* padding to: 8 Bytes */
    uint32_t __padding_0[1];

    /* [0x8]: REG (rw) (comment missing) */
    uint64_t r2;
  } b2;
};
#endif /* !__ASSEMBLER__*/

Note the duplicate __padding_0

This MR fixes the issue by tracking the padding numbering using a stack within CPrinter. The numbering is still reset when a new struct (without omitted prefix) starts.

[tgingold-cern]

Thanks!