RFR: 8293188: x86_64: Introduce stubGenerator_x86_64.hpp

[Vladimir Ivanov]

Introduce stubGenerator_x86_64.hpp and extract StubGenerator class declaration there. 

Testing: hs-tier1 - hs-tier4

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Commit messages:
 - fix
 - stubGenerator_x86_64.hpp

Changes: https://git.openjdk.org/jdk/pull/10111/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10111&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293188
  Stats: 11125 lines in 2 files changed: 3954 ins; 3475 del; 3696 mod
  Patch: https://git.openjdk.org/jdk/pull/10111.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10111/head:pull/10111

PR: https://git.openjdk.org/jdk/pull/10111