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-4.5.2.orig/libjava/Makefile.in gcc-4.5.2/libjava/Makefile.in

--- gcc-4.5.2.orig/libjava/Makefile.in	2010-12-16 13:49:03.000000000 +0100

+++ gcc-4.5.2/libjava/Makefile.in	2010-12-29 23:02:41.000000000 +0100

@@ -9463,6 +9463,9 @@

 ecjx$(EXEEXT): $(ecjx_OBJECTS) $(ecjx_DEPENDENCIES) 

 	@rm -f ecjx$(EXEEXT)

 	$(ecjx_LINK) $(ecjx_OBJECTS) $(ecjx_LDADD) $(LIBS)

+ecjx.$(OBJEXT): $(ecjx_SOURCES)

+	@rm -f ecjx.$(OBJEXT)

+	$(CC_FOR_BUILD) $(BUILD_CFLAGS) -c -o $@ $<

 gappletviewer$(EXEEXT): $(gappletviewer_OBJECTS) $(gappletviewer_DEPENDENCIES) 

 	@rm -f gappletviewer$(EXEEXT)

 	$(gappletviewer_LINK) $(gappletviewer_OBJECTS) $(gappletviewer_LDADD) $(LIBS)