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)

 Original patch from: ../4.3.2/250-sh-pr24836.patch

 -= BEGIN original header =-

 Original patch from gentoo: gentoo/src/patchsets/gcc/4.3.1/gentoo/74_all_sh-pr24836.patch

 http://sourceforge.net/mailarchive/forum.php?thread_id=8959304&forum_id=5348

 http://gcc.gnu.org/PR24836

 -= END original header =-

 diff -durN gcc-4.3.3.orig/gcc/configure gcc-4.3.3/gcc/configure

 --- gcc-4.3.3.orig/gcc/configure	2009-01-27 22:19:18.000000000 +0100

 +++ gcc-4.3.3/gcc/configure	2009-01-27 22:25:33.000000000 +0100

 @@ -20689,7 +20689,7 @@

  	tls_first_minor=14

  	tls_as_opt="-m64 -Aesame --fatal-warnings"

  	;;

 -  sh-*-* | sh[34]-*-*)

 +  sh-*-* | sh[34]*-*-*)

      conftest_s='

  	.section ".tdata","awT",@progbits

  foo:	.long	25

 diff -durN gcc-4.3.3.orig/gcc/configure.ac gcc-4.3.3/gcc/configure.ac

 --- gcc-4.3.3.orig/gcc/configure.ac	2009-01-27 22:19:18.000000000 +0100

 +++ gcc-4.3.3/gcc/configure.ac	2009-01-27 22:25:33.000000000 +0100

 @@ -2552,7 +2552,7 @@

  	tls_first_minor=14

  	tls_as_opt="-m64 -Aesame --fatal-warnings"

  	;;

 -  sh-*-* | sh[34]-*-*)

 +  sh-*-* | sh[34]*-*-*)

      conftest_s='

  	.section ".tdata","awT",@progbits

  foo:	.long	25