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 -urN gcc-4.0.0-100/libstdc++-v3/fragment.am gcc-4.0.0/libstdc++-v3/fragment.am

--- gcc-4.0.0-100/libstdc++-v3/fragment.am	2004-10-25 15:32:40.000000000 -0500

+++ gcc-4.0.0/libstdc++-v3/fragment.am	2005-04-28 21:48:43.000000000 -0500

@@ -18,5 +18,5 @@

 	$(WARN_FLAGS) $(WERROR) -fdiagnostics-show-location=once

 # -I/-D flags to pass when compiling.

-AM_CPPFLAGS = $(GLIBCXX_INCLUDES)

+AM_CPPFLAGS = $(GLIBCXX_INCLUDES) -I$(toplevel_srcdir)/include

diff -urN gcc-4.0.0-100/libstdc++-v3/libmath/Makefile.am gcc-4.0.0/libstdc++-v3/libmath/Makefile.am

--- gcc-4.0.0-100/libstdc++-v3/libmath/Makefile.am	2003-08-27 16:29:42.000000000 -0500

+++ gcc-4.0.0/libstdc++-v3/libmath/Makefile.am	2005-04-28 21:48:43.000000000 -0500

@@ -32,7 +32,7 @@

 libmath_la_SOURCES = stubs.c

-AM_CPPFLAGS = $(CANADIAN_INCLUDES)

+AM_CPPFLAGS = $(CANADIAN_INCLUDES) -I$(toplevel_srcdir)/include

 # Only compiling "C" sources in this directory.

 LIBTOOL = @LIBTOOL@ --tag CC