RFR: 8320826: call allocate_shared_strings_array after all strings are interned

[Ioi Lam]

`StringTable::allocate_shared_strings_array()` creates a Java object array that has the exact same size as the number of currently interned strings. Therefore, we need to move it to just before we enter the safepoint, where we know no more Java code would be execute.

I also had to change the assert in `StringTable::allocate_shared_strings_array()` to a condition check:  it's possible for `HeapShared::init_for_dumping()` to disable heap dumping when a Java agent transforms classes like jdk.internal.math.FDBigInteger.

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Commit messages:
 - fixed whitespaces
 - 8320826: call allocate_shared_strings_array after all strings are interned

Changes: https://git.openjdk.org/jdk/pull/16839/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16839&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320826
  Stats: 8 lines in 2 files changed: 6 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16839.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16839/head:pull/16839

PR: https://git.openjdk.org/jdk/pull/16839