RFR: 8315247: GenShen: Condition calls to post-write barrier code generation by a flag

[Y. Srinivas Ramakrishna]

Protect the card barrier code generation by a new global `ShenandoahCardBarrier`, which is disabled by default and enabled for GenShen (generational mode Shenandoah) only. If the user forces the barrier flag to a value inconsistent with mode, we exit with an appropriate error message.

The changes have been made for x86, aarch, and ppc. I have tested on x86 on a cloud host running jtreg1-4 and SPECjbb. A round of aarch64 and x86 pipeline tests have run (but ran into existing assertion that William is fixing). Will request PPC testing from partners able to test. RISC-V & S390 don't currently support generational mode Shenandoah.

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Commit messages:
 - Check and exit if user specifies a setting of ShenandoahCardBarrier that
 - check that ShenandoahCardBarrier is off by default for most
 - Changes from prelim review by William & Kelvin.
 - Take care of some ppc changes
 - Merge branch 'master' into card_barrier
 - First cut of changes, checking in for testing:

Changes: https://git.openjdk.org/shenandoah/pull/313/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=313&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315247
  Stats: 105 lines in 15 files changed: 47 ins; 21 del; 37 mod
  Patch: https://git.openjdk.org/shenandoah/pull/313.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/313/head:pull/313

PR: https://git.openjdk.org/shenandoah/pull/313