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)

Fix for psim build failure due to -lz

See http://sourceware.org/bugzilla/show_bug.cgi?id=12202

--- gdb-7.2.orig/sim/ppc/Makefile.in

+++ gdb-7.2/sim/ppc/Makefile.in

@@ -551,7 +551,7 @@

 PACKAGE_OBJ = @sim_pk_obj@

-psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBS) $(LIBINTL_DEP)

+psim: $(TARGETLIB) main.o $(LIBIBERTY_LIB) $(BFD_LIB) $(LIBINTL_DEP)

 	$(CC) $(CFLAGS) $(SIM_CFLAGS) $(LDFLAGS) -o psim$(EXEEXT) main.o $(TARGETLIB) $(BFD_LIB) $(LIBINTL) $(LIBIBERTY_LIB) $(LIBS)

 run: psim