debug/ltrace: fix building for mips

ltrace 0.5.3 currently fails to build for target mips because MY_TARGET
(introduced by patches/ltrace/0.5.3/150-allow-configurable-arch.patch)
is set to 'mips' via CT_ARCH, while the mips specific stuff in ltrace
(0.5.3) is stored under sysdeps/linux-gnu/mipsel:

result: *** No rule to make target `mips/arch.h', needed by `sysdep.h'.
Stop.

The following patch fixes this issue

Signed-off-by: "Horst Kronstorfer" <horst.kronstorfer@aon.at>
[yann.morin.1998@anciens.enib.fr: reformat commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 2115901c7f07181d92ddbd7dd7ebeaa72e4fb176)

# 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

    # Add spe in the tuple if needed

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

        glibc,|eglibc,)   CT_TARGET_SYS=gnu;;

        glibc,y|eglibc,y) CT_TARGET_SYS=gnuspe;;

    esac

    # Add extra flags for SPE if needed

    if [ "${CT_ARCH_POWERPC_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

}