[aarch64-port-dev ] RFR: Fix racy update of code roots

[Roman Kennke]

I would like to cherry-pick this critical bug-fix into 
aarch64-port/jdk8-shenandoah:

https://cr.openjdk.java.net/~rkennke/fixracycodecacheupdate/webrev.00/

It fixes a crash that has been observed by a number of users and 
customers. See for example here for details:

https://mail.openjdk.java.net/pipermail/shenandoah-dev/2020-December/014044.html

The bugfix has been in shenandoah/jdk8 for almost a week now and testing 
is clean.

https://mail.openjdk.java.net/pipermail/shenandoah-dev/2020-December/014082.html

It's been confirmed that it does fix the bug. Also, it is not a risky 
fix, it is contained in Shenandoah and once found, it was fairly 
obvious, and it is similar is spirit to what we already do in jdk11+ 
(but slightly different in implementation).

Ok to push this to aarch64-port/jdk8u-shenandoah?

Thanks,
Roman