RFR: 8365558: Fix stub entry init and blob creation on Zero [v3]

[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. this was broken when [JDK-8359373](https://bugs.openjdk.org/browse/JDK-8359373) introduced the preuniverse Stubgen stub group.

Andrew Dinn has updated the pull request incrementally with one additional commit since the last revision:

  relocate  return to correct scope

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26787/files
  - new: https://git.openjdk.org/jdk/pull/26787/files/9a0f0cf0..f33f292d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26787&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26787&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 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