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)

On 20081011, Khem RAJ writes:

 You have to bar gcc trying to build unwinding at stage 1. Try the

 attached patch and see if it helps.

 gcc/config/sh/linux-unwind.h |    3 +++

 1 file changed, 3 insertions(+)

Index: gcc-4_2-branch/gcc/config/sh/linux-unwind.h

===================================================================

--- gcc-4_2-branch.orig/gcc/config/sh/linux-unwind.h	2008-07-01 17:24:40.000000000 -0700

+++ gcc-4_2-branch/gcc/config/sh/linux-unwind.h	2008-07-01 17:25:47.000000000 -0700

@@ -26,6 +26,8 @@ along with this program; see the file CO

 the Free Software Foundation, 51 Franklin Street, Fifth Floor,

 Boston, MA 02110-1301, USA.  */

+#ifndef inhibit_libc

+

 /* Do code reading to identify a signal frame, and set the frame

    state data appropriately.  See unwind-dw2.c for the structs.  */

@@ -251,3 +253,4 @@ sh_fallback_frame_state (struct _Unwind_

   return _URC_NO_REASON;

 }

 #endif /* defined (__SH5__) */

+#endif /* ifndef inhibit_libc */