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.gcc gcc-4.4.0/gcc/config.gcc

--- gcc-4.4.0.orig/gcc/config.gcc	2009-04-17 13:58:41.000000000 +0200

+++ gcc-4.4.0/gcc/config.gcc	2009-05-27 21:38:31.000000000 +0200

@@ -2121,7 +2121,7 @@

 	if test x${sh_multilibs} = x ; then

 		case ${target} in

 		sh64-superh-linux* | \

-		sh[1234]*)	sh_multilibs=${sh_cpu_target} ;;

+		sh[1234]*)	sh_multilibs=`cd ${srcdir}/config/sh ; echo t-mlib-sh[1-4]* | sed 's:t-mlib-sh:,m:g;s: ::g'` ;;

 		sh64* | sh5*)	sh_multilibs=m5-32media,m5-32media-nofpu,m5-compact,m5-compact-nofpu,m5-64media,m5-64media-nofpu ;;

 		sh-superh-*)	sh_multilibs=m4,m4-single,m4-single-only,m4-nofpu ;;

 		sh*-*-linux*)	sh_multilibs=m1,m3e,m4 ;;