RFR: 8286944: Loom: Common ContinuationEntry cookie handling

[Aleksey Shipilev]

Platform-specific code injects the cookie to the head of ContinuationEntry. The value for this cookie is hard-coded in platform-specific code, which is fragile. It would be better to common the cookie handling a bit to ease porting. In fact, I have spent an hour trying to debug a thing where I mistyped a cookie value in x86_32 code, hence my interest.

Additional testing:
 - [x] Linux x86_64 fastdebug, `java/lang/Thread/virtual`
 - [ ] Linux AArch64 fastdebug, `java/lang/Thread/virtual`

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/8762/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8762&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286944
  Stats: 12 lines in 3 files changed: 8 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8762.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8762/head:pull/8762

PR: https://git.openjdk.java.net/jdk/pull/8762