RFR: 8261072: AArch64: Fix MacroAssembler::get_thread convention

[Anton Kozlov]

Please review a fix in a special calling convention for aarch64_get_thread_helper for non-Linux platforms (windows/aarch64 for now). 

Preliminary review: https://mail.openjdk.java.net/pipermail/aarch64-port-dev/2021-January/011239.html

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Commit messages:
 - Fix MacroAssembler::get_thread convention

Changes: https://git.openjdk.java.net/jdk/pull/2451/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2451&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/jdk/pull/2451.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2451/head:pull/2451

PR: https://git.openjdk.java.net/jdk/pull/2451