AArch64: Obsolete UseBarriersForVolatile option

Summary

Obsolete the VM product option UseBarriersForVolatile

Problem

Running application with option "-XX:+UseBarriersForVolatile" might have memory consistent issue like what mentioned in https://bugs.openjdk.java.net/browse/JDK-8241137. Besides it adds complexity to the volatile access implementation, and each usage of it needs to take more consideration of the memory consistent and tests. Since ldar/stlr can work well stably for volatile access, and so does ldaxr/stlxr for unsafe atomic operations, we'd better simplify things by removing this flag and the alternative implementation for volatile memory access.

Solution

Obsolete the option and remove its usages in hotspot.

Here is the discussion details about this flag:

http://mail.openjdk.java.net/pipermail/aarch64-port-dev/2020-March/008679.html

http://mail.openjdk.java.net/pipermail/aarch64-port-dev/2020-April/008706.html

Specification

Running java with option -XX:+/-UseBarriersForVolatile will no longer enable or disable the feature. Instead, it will print the usual obsolete option warning.