SI-8926 default visbility RUNTIME for java annotations

When parsed from source, java annotation class symbol are lacking the
`@Retention` annotation. In mixed compilation, java annotations are
therefore emitted with visibility CLASS.

This patch conservatively uses the RUNTIME visibility in case there is
no @Retention annotation.

The real solution is to fix the Java parser, logged in SI-8928.
Merge pull request #4067 from lrytz/t8926

SI-8926 default visbility RUNTIME for java annotations