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>

 Original patch from: ../crosstool-NG/ct-ng.trunk/patches/binutils/2.19/110-RPATH_ENVVAR-smack.patch

 -= BEGIN original header =-

 Original patch from Gentoo:

 gentoo/src/patchsets/binutils/2.19/08_all_binutils-RPATH_ENVVAR-smack.patch

 http://sourceware.org/ml/binutils/2007-07/msg00401.html

 http://sourceware.org/bugzilla/show_bug.cgi?id=4970

 -= END original header =-

 diff -durN binutils-2.19.1.orig/configure binutils-2.19.1/configure

 --- binutils-2.19.1.orig/configure	2008-09-04 04:18:16.000000000 +0200

 +++ binutils-2.19.1/configure	2009-03-08 11:57:00.000000000 +0100

 @@ -5601,6 +5601,7 @@

    *-*-mingw* | *-*-cygwin ) RPATH_ENVVAR=PATH ;;

    *) RPATH_ENVVAR=LD_LIBRARY_PATH ;;

  esac

 +RPATH_ENVVAR="cant_touch_this_nah_nah_nah"

  # On systems where the dynamic library environment variable is PATH,

  if test "$RPATH_ENVVAR" = PATH; then