RFR: 8266891: Provide a switch to force the class space to a specific location

[Thomas Stuefe]

To test compressed Klass pointer encoding and other metaspace interna, it would be nice to be able to force location of the class space to a specific location. This would help with analysis of problems like JDK-8261552 or JDK-8265705 - errors in Klass* encoding/decoding which managed to stay unnoticed for an astonishingly long time.

For simplicity, it would be sufficient to only have this ability if CDS is disabled (Xshare=off), since CCS allocation is tied to CDS location otherwise and that would be more diffictult to disentangle.

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Commit messages:
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Changes: https://git.openjdk.java.net/jdk/pull/3969/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3969&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266891
  Stats: 58 lines in 2 files changed: 43 ins; 10 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3969.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3969/head:pull/3969

PR: https://git.openjdk.java.net/jdk/pull/3969