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)

 diff -durN binutils-2.20.1.orig/bfd/elf64-ppc.c binutils-2.20.1/bfd/elf64-ppc.c

 --- binutils-2.20.1.orig/bfd/elf64-ppc.c	2010-02-03 14:28:25.000000000 +0100

 +++ binutils-2.20.1/bfd/elf64-ppc.c	2010-08-17 19:32:04.000000000 +0200

 @@ -11991,7 +11991,12 @@

  	      else if (!SYMBOL_REFERENCES_LOCAL (info, &h->elf)

  		       && !is_opd

  		       && r_type != R_PPC64_TOC)

 -		outrel.r_info = ELF64_R_INFO (h->elf.dynindx, r_type);

 +		{

 +		  outrel.r_info = ELF64_R_INFO (h->elf.dynindx, r_type);

 +		  if (h->elf.dynindx == -1

 +		      && h->elf.root.type == bfd_link_hash_undefweak)

 +		  memset (&outrel, 0, sizeof outrel);

 +		}

  	      else

  		{

  		  /* This symbol is local, or marked to become local,