RFR: 8365558: Fix stub entry init and blob creation on Zero

[Andrew Dinn]

Fix two problems with Zero blob/stub entry initialization:
1. Stop Zero allocating AdapterBlobs for the default set of i2c2i adapters even though it cannot ever use them.
2. Allow Zero to execute its stubgen init functions even though it is not generating code into a code buffer (i.e when blob size is declared as zero). This still allows it initialize entries to point to C functions. n.b. yhis was broken when [JDK-8359373](https://bugs.openjdk.org/browse/JDK-8359373) introduced the preuniverse Stubgen stub group.

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Commit messages:
 - avoid  creating AdpaterBlobs on Zero where we cannto use them
 - Execute stubgen stub create functions even on Zero when no code expected

Changes: https://git.openjdk.org/jdk/pull/26787/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26787&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365558
  Stats: 45 lines in 2 files changed: 30 ins; 8 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/26787.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26787/head:pull/26787

PR: https://git.openjdk.org/jdk/pull/26787