Feature: Add ability to avoid source packages from building on specific architectures

Example:

<Source>
    <Name>psyco</Name>
    ..
    ..
    <ExcludeArch>x86_64</ExcludeArch>
</Source>

will avoid the package psyco from building on systems where pisi.conf->architecture == x86_64

pisi-spec.rng

@@ -26,6 +26,9 @@
             <ref name="Name"/>
             <ref name="Homepage"/>
             <ref name="Packager"/>
+            <zeroOrMore>
+                <ref name="ExcludeArch">
+            </zeroOrMore>
             <oneOrMore>
                 <ref name="License"/>
             </oneOrMore>