RFR: 8321640: Move the method barrier_stubs_init from BarrierSetAssembler to BarrierSet

[Guoxiong Li]

Hi all,

 This patch moves the method `barrier_stubs_init` from `BarrierSetAssembler` to `BarrierSet`.
The `BarrierSetAssembler` is an assember which is like `MacroAssembler`, but the method 
`barrier_stubs_init` generates and stores the stubs which is like `StubGenerator`.
So it is not good to place it in `BarrierSetAssembler`.

Thanks for taking the time to review.

Best Regards,
-- Guoixong

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Commit messages:
 - JDK-8321640

Changes: https://git.openjdk.org/jdk/pull/17044/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17044&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321640
  Stats: 19 lines in 8 files changed: 5 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17044.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17044/head:pull/17044

PR: https://git.openjdk.org/jdk/pull/17044