RFR: 8289780: Remove Forte::register_stub

[Ioi Lam]

I removed `Remove Forte::register_stub` since it's used only by the Solaris Forte(TM) Performance Tools collector, which is no longer supported by the JDK.

I also fixed a couple of places where the stub name is computed unnecessarily.

Also renamed some `#ifndef IA64` around the code that I touched.

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Commit messages:
 - 8289780: Remove Forte::register_stub

Changes: https://git.openjdk.org/jdk/pull/9386/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9386&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289780
  Stats: 112 lines in 8 files changed: 6 ins; 102 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9386.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9386/head:pull/9386

PR: https://git.openjdk.org/jdk/pull/9386