MINOR: debug: place a magic pattern at the beginning of post_mortem

MINOR: debug: place a magic pattern at the beginning of post_mortem

In order to ease finding of the post_mortem struct in core dumps, let's
make it start with a recognizable pattern of exactly 32 chars (to
preserve alignment):

  "POST-MORTEM STARTS HERE+7654321\0"

It can then be found like this from gdb:

  (gdb) find 0x000000012345678, 0x0000000100000000, 'P','O','S','T','-','M','O','R','T','E','M'
  0xcfd300 <post_mortem>
  1 pattern found.

Or easier with any other more practical tool (who as ever used "find" in
gdb, given that it cannot iterate over maps and is 100% useless?).