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)

 # Compute x86-specific values

 # This one really needs a little love! :-(

 CT_DoArchTupleValues() {

     # Override the architecture part of the tuple:

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH=x86_64

     else

         arch="${CT_ARCH_ARCH}"

         [ -z "${arch}" ] && arch="${CT_ARCH_TUNE}"

         case "${arch}" in

             "")                           CT_TARGET_ARCH=i386;;

             i386|i486|i586|i686)          CT_TARGET_ARCH="${arch}";;

             winchip*)                     CT_TARGET_ARCH=i486;;

             pentium|pentium-mmx|c3*)      CT_TARGET_ARCH=i586;;

             pentiumpro|pentium*|athlon*)  CT_TARGET_ARCH=i686;;

             prescott)                     CT_TARGET_ARCH=i686;;

             *)                            CT_TARGET_ARCH=i586;;

         esac

     fi

 }