Replace references to COLLATE "en_US" with COLLATE "C".

Robert Haas <rhaas@postgresql.org>

Commit: 71efd34fb8047e3e4fdfc9d6900b7affd20a0235
Author: Robert Haas <rhaas@postgresql.org>
Date: 2016-12-07T18:36:57Z
Releases: 10.0
Replace references to COLLATE "en_US" with COLLATE "C".

Commit f0e44751d7175fa3394da2c8f85e3ceb3cdbfe63 is turning the
buildfarm red; let's try something hopefully more portable.

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