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

 See http://sourceware.org/ml/binutils/2005-12/msg00270.html

 http://sourceware.org/ml/binutils-cvs/2005-12/msg00139.html

 STABS hash table size change, backported from the binutils CVS tree.  Here's

 the CVS log comment for the original change:

 revision 1.25

 date: 2005/12/29 10:29:23;  author: nickc;  state: Exp;  lines: +2 -3

 (_bfd_link_section_stabs): Use bfd_hash_table_init rather than

 bfd_hash_table_init_n(...,251) so that the size of the hash table can be

 controlled by the user.

 --- binutils/bfd/stabs.c.old	4 May 2005 15:53:39 -0000	1.24

 +++ binutils/bfd/stabs.c	29 Dec 2005 10:29:23 -0000	1.25

 @@ -194,9 +194,8 @@

  	goto error_return;

        /* Make sure the first byte is zero.  */

        (void) _bfd_stringtab_add (sinfo->strings, "", TRUE, TRUE);

 -      if (! bfd_hash_table_init_n (&sinfo->includes,

 -				   stab_link_includes_newfunc,

 -				   251))

 +      if (! bfd_hash_table_init (&sinfo->includes,

 +				 stab_link_includes_newfunc))

  	goto error_return;

        sinfo->stabstr = bfd_make_section_anyway (abfd, ".stabstr");

        if (sinfo->stabstr == NULL)