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>

diff -durN binutils-2.21.1.orig/ld/emultempl/elf32.em binutils-2.21.1/ld/emultempl/elf32.em

--- binutils-2.21.1.orig/ld/emultempl/elf32.em	2011-06-27 09:39:14.000000000 +0200

+++ binutils-2.21.1/ld/emultempl/elf32.em	2011-09-12 13:39:08.808908274 +0200

@@ -1272,6 +1272,8 @@

 	      && command_line.rpath == NULL)

 	    {

 	      lib_path = (const char *) getenv ("LD_RUN_PATH");

+	      if ((lib_path) && (strlen (lib_path) == 0))

+	      	lib_path = NULL;

 	      if (gld${EMULATION_NAME}_search_needed (lib_path, &n,

 						      force))

 		break;

@@ -1499,6 +1501,8 @@

   rpath = command_line.rpath;

   if (rpath == NULL)

     rpath = (const char *) getenv ("LD_RUN_PATH");

+  if ((rpath) && (strlen (rpath) == 0))

+  	rpath = NULL;

   for (abfd = link_info.input_bfds; abfd; abfd = abfd->link_next)

     if (bfd_get_flavour (abfd) == bfd_target_elf_flavour)