Clashing build flags for enum sizes cause linker problems

When linking, there is an error about linking from object files using --enum_is_int (ie. stuff built with the Symbian toolchain) to ARM libraries built using --packed_enum.

However, on further study, it seems that the symbols that are causing this linkage problem are actually part of the standard C++ library. As such, they should be linking to the Symbian implementation, not the ARM one. However, due to issues with symbol visibility (see QTBUG-7006), the linker prefers the versions in the ARM-supplied static library over the one's in the Symbian-supplied shared object file (.dso) (reason: the symbols are imported in the object files with STV_HIDDEN visibility, which means that currently the ARM linker won't consider symbols from .dso's as suitable candidates for linking).