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)

# powerpc specific configuration file

config ARCH_powerpc

    select ARCH_SUPPORTS_32

    select ARCH_SUPPORTS_64 if EXPERIMENTAL

    select ARCH_DEFAULT_32

    select ARCH_USE_MMU

    select ARCH_SUPPORT_ABI

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    help

      The PowerPC architecture, as defined by:

        http://www.ibm.com/developerworks/eserver/articles/archguide.html