/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/ is in libgphobos-6-dev-i386-cross 6.4.0-17ubuntu1cross1.
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/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/bits.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/config.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/gc.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/os.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/pooltable.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/proxy.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gc/stats.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/atomics.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/attribute.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/backtrace.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/builtins.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/config.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/deh.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/libbacktrace.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/unwind/
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/unwind/arm.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/unwind/generic.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/unwind/package.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcc/unwind/pe.d
/usr/lib/gcc-cross/i686-linux-gnu/6/include/d/gcstub/gc.d