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)

 http://sources.redhat.com/ml/binutils/2004-04/msg00665.html

 fixes building on some architectures (like m68k/arm/cris/etc...) because

 it does the right thing

 diff -durN glibc-2.12.1.orig/include/libc-symbols.h glibc-2.12.1/include/libc-symbols.h

 --- glibc-2.12.1.orig/include/libc-symbols.h	2009-03-14 00:51:46.000000000 +0100

 +++ glibc-2.12.1/include/libc-symbols.h	2009-11-13 00:50:07.000000000 +0100

 @@ -239,12 +239,12 @@

  #  define __make_section_unallocated(section_string)

  # endif

 -/* Tacking on "\n\t#" to the section name makes gcc put it's bogus

 +/* Tacking on "\n#APP\n\t#" to the section name makes gcc put it's bogus

     section attributes on what looks like a comment to the assembler.  */

  # ifdef HAVE_SECTION_QUOTES

 -#  define __sec_comment "\"\n\t#\""

 +#  define __sec_comment "\"\n#APP\n\t#\""

  # else

 -#  define __sec_comment "\n\t#"

 +#  define __sec_comment "\n#APP\n\t#"

  # endif

  # define link_warning(symbol, msg) \

    __make_section_unallocated (".gnu.warning." #symbol) \