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-4.4.0.orig/gcc/config/netbsd-elf.h gcc-4.4.0/gcc/config/netbsd-elf.h

--- gcc-4.4.0.orig/gcc/config/netbsd-elf.h	2007-09-03 18:14:04.000000000 +0200

+++ gcc-4.4.0/gcc/config/netbsd-elf.h	2009-05-27 21:38:14.000000000 +0200

@@ -82,6 +82,7 @@

 #define NETBSD_LINK_SPEC_ELF \

   "%{assert*} %{R*} %{rpath*} \

    %{shared:-shared} \

+   %{symbolic:-Bsymbolic} \

    %{!shared: \

      -dc -dp \

      %{!nostdlib: \