/usr/lib/gcc-cross/m68k-linux-gnu/6/include/ is in libgcc-6-dev-m68k-cross 6.4.0-17ubuntu1cross1.
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/usr/lib/gcc-cross/m68k-linux-gnu/6/include-fixed/README
/usr/lib/gcc-cross/m68k-linux-gnu/6/include-fixed/limits.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include-fixed/syslimits.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/float.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/iso646.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/math-68881.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/omp.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/openacc.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdalign.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdarg.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdatomic.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdbool.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stddef.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdfix.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdint-gcc.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdint.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/stdnoreturn.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/unwind.h
/usr/lib/gcc-cross/m68k-linux-gnu/6/include/varargs.h