Java 15 toolchain support

[davido]

Description of the problem / feature request:

Support for java 15 as --java_toolchain and --host_java_toolchain.

Feature requests: what underlying problem are you trying to solve with this feature?

Build Java code and produce major java version 15.

The release schedule for Java 15 is here: [1]

• 2020/08/06 Initial Release Candidate
• 2020/08/20 Final Release Candidate
• 2020/09/15 General Availability

Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

  $ bazel build \
     --java_toolchain=@remote_java_tools_linux//:toolchain_jdk_15 \
     --host_java_toolchain=@remote_java_tools_linux//:toolchain_jdk_15 \
     --javabase=@openjdk15_linux_archive//:runtime \
     --host_javabase=@openjdk15_linux_archive//:runtime \
   :foo

[1] https://openjdk.java.net/projects/jdk/15

[c-mita]

cc @comius