scripts: add option to start an interactive debug shell

Add an option that, when a command fails:
- starts an interactive shell with the failed command's environment
- attempts re-execution of the failed command, continues, or aborts
at user's whim.

Before starting the debug-shell, the backtrace is printed.
When exiting for an abort, the standard error message is printed.

Based on an idea and a patch from: Johannes Stezenbach <js@sig21.net>
http://sourceware.org/ml/crossgcc/2012-09/msg00144.html

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[yann.morin.1998@free.fr: integrate in the fault handler]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Johannes Stezenbach <js@sig21.net>
Patchwork-Id: 191571
Patchwork-Id: 191668

# 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_ENDIAN}" in

        big)    CT_ARCH_ENDIAN_CFLAG="-EB";;

        little) 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}"

}