RFR: 8286944: Loom: Common ContinuationEntry cookie handling

[Aleksey Shipilev]

On Wed, 18 May 2022 09:46:43 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> 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`
>  - [x] Linux AArch64 fastdebug, `java/lang/Thread/virtual`

I retested both x86_64 and AArch64 with new testgroups, and the tests are passing.

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