RFR: 8258490: Shenandoah: Full GC does not need to remark threads and drain SATB buffers

[Zhengyu Gu]

Full GC marks heap at a pause with SATB deactivated, therefore, we don't need to remark threads and drain SATB buffers during final mark phase.

- [x] hotspot_gc_shenandoah

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

Commit messages:
 - Silent MacOSX build
 - JDK-8258490

Changes: https://git.openjdk.java.net/jdk/pull/1805/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1805&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258490
  Stats: 45 lines in 1 file changed: 22 ins; 15 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1805.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1805/head:pull/1805

PR: https://git.openjdk.java.net/jdk/pull/1805