[15] RFR 8241765: Shenandoah: AARCH64 need to save/restore call clobbered registers before calling keepalive barrier

[Zhengyu Gu]

This bug was discovered while testing aarch64 nmethod entry barrier 
patch posted by Stuart Monteith [1].

We had the same issues on x86 platforms, and fixed by JDK-8233500 and 
JDK-8237776, but never fixed aarch64.

Bug: https://bugs.openjdk.java.net/browse/JDK-8241765
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8241765/webrev.00/

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu

[1] 
https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2020-March/028998.html