RFR: Bring genshen up to date with master

[earthling-amzn]

On Fri, 12 Feb 2021 18:20:29 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Brought mainline (master) commits in a series of smaller merges to make the conflicts more manageable. The refactoring of concurrent mark and the control thread required some non-trivial conflict resolution. I've tested `satb` mode and `generational` mode on the dacapo suite.
>
> This is impossible to actually review, so I'm limited to testing it:
> it finds the following compilation failure. It's probably lacking in include there. Make sure to test with --disable-precompiled-headers.
> 
> === Output from failing command(s) repeated here ===
> * For target hotspot_variant-server_libjvm_objs_BUILD_LIBJVM_link:
> 
> 
> collect2: error: ld returned 1 exit status
> 
> * All command lines available in /home/rkennke/src/openjdk/shenandoah/build/linux-x86_64-server-fastdebug/make-support/failure-logs.
> === End of repeated output ===

Taking a look now. The bot suggests changing the title of this PR to indicate that it's a merge, otherwise it will be squashed. If this gets squashed, won't it make subsequent merges from master more difficult? Is it okay with you if I change the title of this PR so this goes into a `genshen` as a merge and not a giant squashed commit?

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PR: https://git.openjdk.java.net/shenandoah/pull/14