RFR: 8287233: Crash in Continuation.enterSpecial: stop: tried to execute native method as non-native

[Ron Pressler]

Please review this fix to a continuation-related crash that happens due to a race between the linking and generation of Continuation.enterSpecial.

Passes Loom tiers 1-5.

-------------

Commit messages:
 - Revert "Move ContinuationEntry::set_enter_nmethod to set_code to be called under a lock"
 - Properly initialized Continuation.enterSpecial entry points
 - Move ContinuationEntry::set_enter_nmethod to set_code to be called under a lock

Changes: https://git.openjdk.java.net/jdk/pull/8926/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8926&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287233
  Stats: 24 lines in 2 files changed: 14 ins; 3 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8926.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8926/head:pull/8926

PR: https://git.openjdk.java.net/jdk/pull/8926