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)

 Enable building a pure soft-float compiler without the need for a software

 floating point library.

 diff -dur gcc-4.0.4.orig/gcc/config.gcc gcc-4.0.4/gcc/config.gcc

 --- gcc-4.0.4.orig/gcc/config.gcc	2007-02-02 19:12:28.000000000 +0100

 +++ gcc-4.0.4/gcc/config.gcc	2007-02-02 19:12:07.000000000 +0100

 @@ -677,7 +677,7 @@

  		tm_defines="TARGET_BIG_ENDIAN_DEFAULT=1 $tm_defines"

  		;;

  	esac

 -	tmake_file="${tmake_file} arm/t-arm arm/t-linux"

 +	tmake_file="${tmake_file} arm/t-arm arm/t-linux arm/t-arm-elf"

  	extra_parts="crtbegin.o crtbeginS.o crtend.o crtendS.o"

  	gnu_ld=yes

  	;;