toolchaintechnotes: add the rationale for cross compiling ch6 packages

"need to be cross compiled" alone does not make too much sense: we
compile these packages in chapter 8 anyway.  The real reason forcing a
cross compilation is circular dependency: if building A needs B but
building B needs A, we'll have to cross compile at least one of A and B
or we won't be able to build either in the chroot.

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