RFR: 8294033: x86_64: libm stubs are missing

Fri Nov 11 02:24:21 UTC 2022

There's a regression from [JDK-8293285](https://bugs.openjdk.org/browse/JDK-8293285) refactoring where I forgot to call generate_libm_stubs() during stub initialization phase.

The patch restores proper stub init sequence and also piles some minor refactorings on top. 

Testing: hs-tier1 - hs-tier2

Thanks!

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Commit messages:
 - 8294033: x86_64: libm stubs are missing

Changes: https://git.openjdk.org/jdk/pull/11100/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11100&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294033
  Stats: 25 lines in 2 files changed: 6 ins; 9 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/11100.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11100/head:pull/11100

PR: https://git.openjdk.org/jdk/pull/11100