[jdk17u] RFR: JDK-8274795: AArch64: avoid spilling and restoring r18 in macro assembler

[Bernhard Urban-Forster]

Clean backport of [JDK-8274795](https://bugs.openjdk.java.net/browse/JDK-8274795) / https://github.com/openjdk/jdk/commit/ede3f4e94c752a8457b7c24e001bd122845d2f6a

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Commit messages:
 - Backport ede3f4e94c752a8457b7c24e001bd122845d2f6a

Changes: https://git.openjdk.java.net/jdk17u/pull/197/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=197&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274795
  Stats: 25 lines in 3 files changed: 8 ins; 13 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/197.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/197/head:pull/197

PR: https://git.openjdk.java.net/jdk17u/pull/197