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/2.19/180-libiberty-pic.patch

 -= BEGIN original header =-

 Original patch from Gentoo:

 gentoo/src/patchsets/binutils/2.19/91_all_libiberty-pic.patch

 -= END original header =-

 diff -durN binutils-2.19.1.orig/libiberty/Makefile.in binutils-2.19.1/libiberty/Makefile.in

 --- binutils-2.19.1.orig/libiberty/Makefile.in	2008-07-24 15:51:49.000000000 +0200

 +++ binutils-2.19.1/libiberty/Makefile.in	2009-03-08 11:57:03.000000000 +0100

 @@ -227,6 +227,7 @@

  	  $(AR) $(AR_FLAGS) $(TARGETLIB) \

  	    $(REQUIRED_OFILES) $(EXTRA_OFILES) $(LIBOBJS); \

  	  $(RANLIB) $(TARGETLIB); \

 +	  cp $(TARGETLIB) ../ ; \

  	  cd ..; \

  	else true; fi