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 ARM-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${target_endian_eb}"

     # The system part of the tuple:

     case "${CT_LIBC},${CT_ARCH_ARM_EABI}" in

         *glibc,y)   CT_TARGET_SYS=gnueabi;;

         uClibc,y)   CT_TARGET_SYS=uclibcgnueabi;;

         *,y)        CT_TARGET_SYS=eabi;;

     esac

     # Set the default instruction set mode

     case "${CT_ARCH_ARM_MODE}" in

         arm)    ;;

         thumb)

             CT_ARCH_CC_CORE_EXTRA_CONFIG="--with-mode=thumb"

             CT_ARCH_CC_EXTRA_CONFIG="--with-mode=thumb"

 #            CT_ARCH_TARGET_CFLAGS="-mthumb"

             ;;

     esac

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

         CT_ARCH_TARGET_CFLAGS+=" -mthumb-interwork"

     fi

 }