RFR: Bring genshen up to date with master

[Roman Kennke]

On Fri, 12 Feb 2021 17:44:24 GMT, earthling-amzn <github.com+71722661+earthling-amzn 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 ===

-------------

Changes requested by rkennke (Reviewer).

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