RFR(XS) 8248666: AArch64: Use THREAD_LOCAL instead of __thread

[Bernhard Urban-Forster]

Hi all,

a trivial code cleanup to use an existing macro in the AArch64 backend.

JBS: https://bugs.openjdk.java.net/browse/JDK-8248666
Webrev: http://cr.openjdk.java.net/~burban/8248666/


Thank you,
-Bernhard