RFR: [8054714] Use StringJoiner where it makes the code cleaner

Hello everyone! This is a follower of the recently fixed [8051382] -- Optimize java.lang.reflect.Modifier.toString(). I found a dozen+ other places in jdk, where using the StringJoiner makes the code shorter and cleaner. Would you please help review the fix? http://cr.openjdk.java.net/~igerasim/8054714/0/webrev/ Sincerely yours, Ivan