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 gcc-4.4.5.orig/include/libiberty.h gcc-4.4.5/include/libiberty.h

--- gcc-4.4.5.orig/include/libiberty.h	2008-06-24 03:42:31.000000000 +0200

+++ gcc-4.4.5/include/libiberty.h	2010-10-09 23:01:30.000000000 +0200

@@ -595,8 +595,11 @@

 /* Like sprintf but provides a pointer to malloc'd storage, which must

    be freed by the caller.  */

+/* asprintf may be declared as a macro by glibc with __USE_FORTIFY_LEVEL.  */

+#ifndef asprintf

 extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;

 #endif

+#endif

 #if !HAVE_DECL_VASPRINTF

 /* Like vsprintf but provides a pointer to malloc'd storage, which