RFR: 8287071: Loom: Cleanup x86_64 gen_continuation_enter [v5]

[Aleksey Shipilev]

> During x86_32 porting, I ended up rewriting the copy of x86_64 gen_continuation_enter significantly to make it more streamlined. The original x86_64 gen_continuation_enter should enjoy the similar streamlining, since it would likely to be the template for many other architectures.
> 
> There might be more things to do, I would not know until x86_32 fully works. I am pretty sure current thing is self-sufficient.
> 
> Brief tour of changes:
>  - Check the incoming registers explicitly
>  - Pass explicit argument registers to `fill_continuation_entry`
>  - Use the usual `NativeCall` alignment code to produce a patchable call
>  - Check that static stub is emitted, fail hard otherwise
>  - Handle register shuffling for `exception_handler_for_return_address` the similar way we do it in `StubGenerator`
> 
> Additional testing:
>  - [x] Linux x86_64 fastdebug, prospective `jdk_loom hotspot_loom`

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Review feedback

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8802/files
  - new: https://git.openjdk.java.net/jdk/pull/8802/files/3c721970..80960e2a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8802&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8802&range=03-04

  Stats: 6 lines in 1 file changed: 2 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8802.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8802/head:pull/8802

PR: https://git.openjdk.java.net/jdk/pull/8802