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/2.19/170-use-relro.patch

 -= BEGIN original header =-

 Original patch from Gentoo:

 gentoo/src/patchsets/binutils/2.19/78_all_use-relro.patch

 -= END original header =-

 diff -durN binutils-2.19.1.orig/ld/ldmain.c binutils-2.19.1/ld/ldmain.c

 --- binutils-2.19.1.orig/ld/ldmain.c	2009-03-08 11:57:03.000000000 +0100

 +++ binutils-2.19.1/ld/ldmain.c	2009-03-08 11:57:03.000000000 +0100

 @@ -264,6 +264,7 @@

    link_info.keep_memory = TRUE;

    link_info.new_dtags = TRUE;

    link_info.combreloc = TRUE;

 +  link_info.relro = TRUE;

    link_info.strip_discarded = TRUE;

    link_info.emit_hash = TRUE;

    link_info.callbacks = &link_callbacks;