Building Qt for Android with -ltcg fails to compile

With Android, we seem to unconditionally optimize for size (at least in releas builds); and thus passing -Oz along. However, -Oz isn't an option that the linker understands (at least until https://reviews.llvm.org/D79919 lands). As we pass the option to the linker when configured with -ltcg, the build breaks.

Note that this was also broken in Qt 5 with qmake for basically the same reason.