Add JDK 8 tools.jar contents to bootclasspath

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Work towards bazelbuild/bazel#21771

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@cushon Could you review this?

CC @rbeasley

[cushon]

Thanks, this LGTM.

I have one observation, I am not requesting a change: inside Google I've done some work to restrict the set of JDK APIs that are available by default to only include the [root modules] for JDK 9 and newer, and to also omit internal JDK APIs from the bootclasspath jar.

For Bazel I could see some value in discouraging accidentally using internal / non-root / non-default APIs, including tools.jar. But it would need to be possible to use those APIs for builds that really wanted them, and I don't have great ideas for making that optional; we already have a lot of configuration knobs.

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In terms of knobs, as it's essentially a portability issue, maybe this could become a new errorprone check that could then be configured on the toolchain/package/target level as usual?