* option to customize arguments passed to linker

[dkimitsa]

this is being asked time-to-time. usually is is not required as can be covered by robovm, however it might be required for custom builds. to be configured using robovm.xml, sample config:

<config>

    <tools>
        <linker>
            <flags>
                <flag>-fembed-bitcode</flag>
                <flag>--other-flag</flag>
            </flags>
        </linker>
    </tools>
</config>