RFR: 8293036: x86_64: Move Continuation-related stub methods to MacroAssembler

[Vladimir Ivanov]

Move Continuation-related methods from `stubGenerator_x86_64.cpp` under `MacroAssembler`.

Testing: hs-tier1 - hs-tier2

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Commit messages:
 - include
 - fix
 - move

Changes: https://git.openjdk.org/jdk/pull/10066/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10066&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293036
  Stats: 238 lines in 4 files changed: 126 ins; 105 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/10066.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10066/head:pull/10066

PR: https://git.openjdk.org/jdk/pull/10066