[jdk11u-dev] RFR: 8261072: AArch64: Fix MacroAssembler::get_thread convention

[Reka Kovacs]

Clean backport. The non-Linux path will only be exercised once [JEP 388](https://github.com/openjdk/jdk11u-dev/pull/222) is in.

This is part of the Windows/AArch64 port.

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Commit messages:
 - Backport b670efd896a2905b6842b8199261aa2f3d7c12b9

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/271/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=271&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261072
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/271.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/271/head:pull/271

PR: https://git.openjdk.java.net/jdk11u-dev/pull/271