scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

 Signed-off-by: dank@kegel.com

 Raising the size of the hash table is a noticable win when linking 

 at least one large app.  

 There was a patch,

 http://sources.redhat.com/ml/binutils/2004-06/msg00165.html

 to do this dynamically.  No idea why that didn't make it in.

 So this tiny change to raise the default size is just

 a stopgap for now.

 --- binutils-2.16.1/bfd/hash.c.old	2006-03-02 23:01:39.000000000 -0800

 +++ binutils-2.16.1/bfd/hash.c	2006-03-02 23:01:58.000000000 -0800

 @@ -300,7 +300,7 @@

  */

  /* The default number of entries to use when creating a hash table.  */

 -#define DEFAULT_SIZE 4051

 +#define DEFAULT_SIZE 32749

  static size_t bfd_default_hash_table_size = DEFAULT_SIZE;

  /* Create a new hash table, given a number of entries.  */