Using -Oz causes crash, revert to Os

This is with the Xcode 5.1.1 tool-chain. Either a compiler bug, or
some undefined behaviour in our code sensitive to compiler
optimisation details.

The crash we noticed was in sw/source/core/layout/paintfrm.cxx. (There
might of course be more locations with similar behaviour.)

Change-Id: I5d0f0aee369b2ca5fc77356f0a7cb88bec6cc58e