Return archs of self.ctx that are valid build archs
        for the Recipe.

def filtered_archs(self):
        '''Return archs of self.ctx that are valid build archs
        for the Recipe.'''
        result = []
        for arch in self.ctx.archs:
            if not self.archs or (arch.arch in self.archs):
                result.append(arch)
        return result