Reduce platform dependant selects in //tools/jdk/BUILD

[comius]

Macro default_java_toolchain used to create a java_toolchain rule with each attribute set using a platform dependent select. For example ijar attribute was set to //tools/jdk:ijar, which was a macro remote_java_tools_filegroup. That macro contains a select choosing the right @remote_java_tools repository.

With Java toolchainisation this become a fragile mechanism (selects need to match the toolchain...), which would be hard to maintain.

In previous commits java_toolchain_default was implemented, which is part of each java_tools repository and does not use any selects.

For backward compatibility default_java_toolchain was reimplemented to invoke java_toolchain_default. Implementation is not perfect, because it load 3 java_tools repositories (linux,darwin,windows). Not to invoke this misfeature, some macros were moved into utils.bzl. The use of default_java_toolchain should be discouraged.

remote_java_tools_filegroup/import targets in //tools/jdk/BUILD that were previous used only by default_java_toolchain were removed.

default_java_toolchain targets in //tools/jdk/BUILD were replaced with select aliases. Those will be removed when Java toolchainsation is complete.

legacy_toolchain was removed as a fall-back - it was an awkard one - going from Java 11 to Java 8 language level.

[comius]

Downstream tests at https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/1779
Failing gerrit, because of labels used in default_java_toolchain.