[15] RFR 8244420: Shenandoah: Ensure _disarmed_value offset < 128

[Zhengyu Gu]

Please review this small patch that ensures per-thread's disarmed_value 
offset is under 126 bytes. The offset is encoded in nmethod entry 
barrier code stream as a disp8 on x86_64.

Bug: https://bugs.openjdk.java.net/browse/JDK-8244420
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8244420/webrev.00/

Test:
   hotspot_gc_shenandoah


Thanks,

-Zhengyu