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?).

src/debug.c

@@ -98,6 +98,7 @@ struct post_mortem_component {
  */
 struct post_mortem {
 	/* platform-specific information */
+	char post_mortem_magic[32];     // "POST-MORTEM STARTS HERE+7654321\0"
 	struct {
 		struct utsname utsname; // OS name+ver+arch+hostname
 		char hw_vendor[64];     // hardware/hypervisor vendor when known
@@ -2511,6 +2512,10 @@ static void feed_post_mortem_linux()
 
 static int feed_post_mortem()
 {
+	/* write an easily identifiable magic at the beginning of the struct */
+	strncpy(post_mortem.post_mortem_magic,
+		"POST-MORTEM STARTS HERE+7654321\0",
+		sizeof(post_mortem.post_mortem_magic));
 	/* kernel type, version and arch */
 	uname(&post_mortem.platform.utsname);