[AddonManager] - add support for <architecture> tag in addons.xml - addo...

...ns can mark support based on cpu arch (all, x86_32, x86_64, arm_32, arm_64, mips_32, mips_64) - if tag is not found - any arch is assumed to be supported by the addon.

We will need this for binary addons ...

this also fixes a "todo" in the code as you can see in the diff ;)