[13u] RFR: 8229022: BufferedReader performance can be improved by using StringBuilder

[Sergey Nazarkin]

Original change:
https://bugs.openjdk.java.net/browse/JDK-8229022
https://hg.openjdk.java.net/jdk/jdk/rev/ed0058d06107

Simple on-liner improves performance notably. The changes is part of jdk14 and going to be part to jdk11 and jdk8. 

webrev
http://cr.openjdk.java.net/~snazarki/webrev/8229022/

The patch is not applied cleanly due to context change. 

Sergey Nazarkin