ios: correctly renames main

we still use the #define for this, which might change also unrelated
uses of main. We might want to use a pre link step to rename the
mangled symbol as done by mkspecs/macx-ios-clang/rename_main.sh

Change-Id: Ib013a506109ad472bdc202c9470767493ad5cdcb
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>