[11u] RFR: 8239787: AArch64: String.indexOf may incorrectly handle empty strings.

[Alexey Bakhtin]

Original bug:
https://bugs.openjdk.java.net/browse/JDK-8239787
https://hg.openjdk.java.net/jdk/jdk/rev/a3a462ce27cd

Please, review this patch backporting JDK-8239787 to JDK 11u. The patch applies rather cleanly with only conflict in macroAssembler_aarch64.cpp caused by strict type cast added by JDK-8206895 in JDK13

11u webrev: http://cr.openjdk.java.net/~dcherepanov/openjdk11/8239787/webrev.v0/

Thank you
Alexey