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 powerpc-specific values

CT_DoArchTupleValues () {

    # The architecture part of the tuple, override only for 64-bit

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

        CT_TARGET_ARCH="powerpc64"

    fi

    # Only override values when ABI is not the default

    case "${CT_ARCH_powerpc_ABI}" in

        eabi)

            # EABI is only for bare-metal, so libc ∈ [none,newlib]

            CT_TARGET_SYS="eabi"

            ;;

        spe)

            case "${CT_LIBC}" in

                none|newlib)    CT_TARGET_SYS="spe";;

                *glibc)         CT_TARGET_SYS="gnuspe";;

                uClibc)         CT_TARGET_SYS="uclibcgnuspe";;

            esac

            ;;

    esac

    # Add extra flags for SPE if needed

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

        CT_ARCH_TARGET_CFLAGS="-mabi=spe -mspe"

        CT_ARCH_CC_CORE_EXTRA_CONFIG="--enable-e500_double"

        CT_ARCH_CC_EXTRA_CONFIG="--enable-e500_double"

    fi

}