Use load() when constructing QSimpleCodec's reverse map

Use load() when constructing QSimpleCodec's reverse map

The implicit casting results in unnecessary volatile loads. The
test-and-set-ordered doesn't need full ordering, test-and-set-release
is enough to ensure that the memory initialization done by
buildReverseMap() and test-and-set happen in program order.

Change-Id: I168b504271aeba050d6b8396becbdeb3ef938213
Reviewed-by: Olivier Goffart <ogoffart@kde.org>