RFR: 8268174: Move x86-specific stub declarations into stubRoutines_x86.hpp

[Vladimir Ivanov]

A cleanup which makes x86-specific stubs platform-specific.

Testing:
- [x] hs-tier1 - hs-tier2
- [x] x86_32 hotspot build

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Commit messages:
 - Build fix
 - Move x86-specific entries into stubRoutines_x86.hpp

Changes: https://git.openjdk.java.net/jdk/pull/4335/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4335&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268174
  Stats: 110 lines in 16 files changed: 25 ins; 31 del; 54 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4335.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4335/head:pull/4335

PR: https://git.openjdk.java.net/jdk/pull/4335