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)

 # Menu for the cross GDB

 config GDB_CROSS

     bool

     prompt "Cross-gdb"

     default y

     select GDB_GDBSERVER if ! BARE_METAL

     help

       Build and install a cross-gdb for the target, to run on host.

 if GDB_CROSS

 config GDB_CROSS_STATIC

     bool

     prompt "Build a static cross gdb"

     default n

     help

       A static cross gdb can be usefull if you debug on a machine that is

       not the one that is used to compile the toolchain.

       That way, you can share the cross-gdb without installing a toolchain

       on every machine that will be used to debug target programs.

 endif # GDB_CROSS