[gccgo] Make sure go-system.h is always included first

[Ian Lance Taylor]

This gccgo patch makes sure that go-system.h is always the first header
file included.  go-system.h in turn includes config.h first, so this
ensure that config.h always comes before anything else, as required for
large file support.  In order to do this, I added includes of <list> and
<algorithm> to go-system.h.  I also realized that there is one use of
unordered_set in the frontend, so I handles that like unordered_map.
Committed to gccgo branch.

Ian

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