Revert "Update to GCC 7.1 for stage1 and stage2 bootstrap"

Javier Jardónrequested to merge
staging/jjardon/revert_gcc71 into master

Im all for this change, but this was merged but without passing the whole CI; current CI is broken atm because this change (we need to upgrade several old chunks first)

This reverts commit 64813d01.

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