debug/gdb: install ncurses in a temporary place

We only build the static ncurses, to be used to build the native gdb,
and it needs not be available for anyone but us. So install it into
a temporary place, and get rid of it once gdb is built.

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

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

         CT_TARGET_ARCH="s390x"

     fi

 }