RFR: 8259350: Add some internal debugging APIs to the debug agent

[Chris Plummer]

On Thu, 7 Jan 2021 04:28:19 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> This PR adds some APIs that are useful when debugging the debug agent. They can be called from gdb or from other parts of the debug agent. They mostly do things like dumping internal data structures that are tedious to dump or iterate over in gdb. I developed them while working on loom and found them useful.
> 
> Note that `dumpThreadList()` and `dumpThread()` are not exported from threadControl.c because the argument types are only visible within threadControl.c.
> 
> I debated whether to bracket all these APIs with `#ifdef DEBUG`. In the end I decided to in order to make it clear they are only meant for debugging purposes. If you temporarily need them with a product build, you can always modify the code to include them. I could be persuaded `#ifdef DEBUG` though.

Ping! Still need 2 reviews.

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PR: https://git.openjdk.java.net/jdk/pull/1970