RFR [8051382] Optimize java.lang.reflect.Modifier.toString()

[Ivan Gerasimov]

Hello!

StringBuilder can be replaced with StringJoiner, which will make the 
code shorter and, probably, more efficient.
The existing regtest (java/lang/reflect/Modifier/toStringTest.java) can 
be used to make sure nothing is broken.

Would you please help review the fix?

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8051382
WEBREV: http://cr.openjdk.java.net/~igerasim/8051382/0/webrev/

Sincerely yours,
Ivan