RFR: 8278479: RunThese test failure with +UseHeavyMonitors and +VerifyHeavyMonitors

[Coleen Phillimore]

This change adds a null check before calling into Runtime1::monitorenter when -XX:+UseHeavyMonitors is set. There's a null check in the C2 and interpreter code before calling the runtime function but not C1.

Tested with tier1-7 (a little of 8) and built on most non-oracle platforms as well.

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Commit messages:
 - Fix aarch64 overloading to get the right null check.
 - Revert UseHeavyMonitors setting
 - 8278479: RunThese test failure with +UseHeavyMonitors and +VerifyHeavyMonitors

Changes: https://git.openjdk.org/jdk/pull/9339/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9339&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8278479
  Stats: 30 lines in 6 files changed: 30 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9339.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9339/head:pull/9339

PR: https://git.openjdk.org/jdk/pull/9339