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

[Ivan Gerasimov]

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