RFR: 8274738: ZGC: Use relaxed atomic load when reading bits in the live map

[Per Liden]

`ZLiveMap::get()` currently uses a non-atomic load to read bits in the live map. This should really be an relaxed atomic load.

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Commit messages:
 - 8274738: ZGC: Use relaxed atomic load when reading bits in the live map

Changes: https://git.openjdk.java.net/jdk/pull/5809/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5809&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274738
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5809.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5809/head:pull/5809

PR: https://git.openjdk.java.net/jdk/pull/5809