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>

--- binutils-2.20/ld/configure.tgt.orig	2010-02-20 04:05:31.000000000 -0500

+++ binutils-2.20/ld/configure.tgt	2010-02-20 04:08:11.000000000 -0500

@@ -400,11 +400,11 @@

 mips*-*-vxworks*)	targ_emul=elf32ebmipvxworks

 		        targ_extra_emuls="elf32elmipvxworks" ;;

 mips*-*-windiss)	targ_emul=elf32mipswindiss ;;

-mips64*el-*-linux-*)	targ_emul=elf32ltsmipn32

-			targ_extra_emuls="elf32btsmipn32 elf32ltsmip elf32btsmip elf64ltsmip elf64btsmip"

+mips64*el-*-linux-*)	targ_emul=elf64ltsmip

+			targ_extra_emuls="elf32ltsmipn32 elf32btsmipn32 elf32ltsmip elf32btsmip elf64btsmip"

 			targ_extra_libpath=$targ_extra_emuls ;;

-mips64*-*-linux-*)	targ_emul=elf32btsmipn32

-			targ_extra_emuls="elf32ltsmipn32 elf32btsmip elf32ltsmip elf64btsmip elf64ltsmip"

+mips64*-*-linux-*)	targ_emul=elf64btsmip

+			targ_extra_emuls="elf32btsmipn32 elf32ltsmipn32 elf32btsmip elf32ltsmip elf64ltsmip"

 			targ_extra_libpath=$targ_extra_emuls ;;

 mips*el-*-linux-*)	targ_emul=elf32ltsmip

 			targ_extra_emuls="elf32btsmip elf32ltsmipn32 elf64ltsmip elf32btsmipn32 elf64btsmip"