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

}