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 MIPS-specific values

CT_DoArchTupleValues() {

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

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

        CT_TARGET_ARCH="mips64${target_endian_el}"

    else

        # The architecture part of the tuple:

        CT_TARGET_ARCH="${CT_ARCH}${target_endian_el}"

    fi

    # Override CFLAGS for endianness:

    case "${CT_ARCH_BE},${CT_ARCH_LE}" in

        y,) CT_ARCH_ENDIAN_CFLAG="-EB";;

        ,y) CT_ARCH_ENDIAN_CFLAG="-EL";;

    esac

    # Override ABI flags

    CT_ARCH_ABI_CFLAG="-mabi=${CT_ARCH_mips_ABI}"

    CT_ARCH_WITH_ABI="--with-abi=${CT_ARCH_mips_ABI}"

}