[lworld] RFR: 8373346: [lworld] Link error on x64 with minimal build

[Stefan Karlsson]

Minimal builds on x86 fail because some new functions have been defined inside a COMPILER2_OR_JVMCI block. I've moved the code so that the three `clear_mem` functions are kept together and alone in their COMPILER2_OR_JVMCI block.

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Commit messages:
 - Fix minimal x86

Changes: https://git.openjdk.org/valhalla/pull/1780/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1780&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373346
  Stats: 142 lines in 1 file changed: 71 ins; 71 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1780.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1780/head:pull/1780

PR: https://git.openjdk.org/valhalla/pull/1780