RFR: 8295225: [JVMCI] codeStart should be cleared when entryPoint is cleared

[Doug Simon]

When the `InstalledCode.entryPoint` field is [cleared](https://github.com/openjdk/jdk/search?q=set_InstalledCode_entryPoint), the `HotSpotInstalledCode.codeStart` field should also be cleared. That is, when making an nmethod non-entrant, all Java fields pointing to code in the nmethod should be cleared. This avoids an inconsistent view of the code.

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Commit messages:
 - clear codeStart when entryPoint is cleared

Changes: https://git.openjdk.org/jdk/pull/10682/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10682&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295225
  Stats: 6 lines in 3 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10682.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10682/head:pull/10682

PR: https://git.openjdk.org/jdk/pull/10682