Reference `platformclasspath` in `@bazel_tools`

The `platformclasspath` rule in `@rules_java` always requires a
bootstrap Java runtime toolchain to be registered, but whether the
commonly used toolchain macros in `@bazel_tools` register this
toolchain depends on the version of Bazel that is running the build
and the version of `rules_java` it bundles as `@rules_java_builtin`.

In order to support all Bazel versions, reference the
`platformclasspath` in `@bazel_tools`, which in turn references the
corresponding target in `@rules_java_builtin`, which may or may not be
the current version of this code.