[jdk11u-dev] RFR: 8248500: AArch64: Avoid usage of r18

[Bernhard Urban-Forster]

The ABI for AArch64 reserves r18 as a platform register [1]. Linux doesn't make use of it, thus it's available as an additional callee-saved register. This is not the case for Windows and macOS, therefore we have to make HotSpot aware of that.

[1] https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#61machine-registers

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Commit messages:
 - AArch64: Avoid usage of r18

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/567/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=567&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248500
  Stats: 1373 lines in 13 files changed: 125 ins; 3 del; 1245 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/567.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/567/head:pull/567

PR: https://git.openjdk.java.net/jdk11u-dev/pull/567