RFR(S): 8246373 AArch64: Refactor register spilling code in ZGC barriers

[Stuart Monteith]

Hello,
    There is an opportunity to spill/load fewer registers in ZGC barriers. This is targetting JDK 16 jdk/jdk.

JIRA:
	https://bugs.openjdk.java.net/browse/JDK-8246373
Webrev:
	http://cr.openjdk.java.net/~smonteith/8246373/webrev.0/

Tested with JTReg, SPECjbb2015, and other workloads.

Thanks,
	Stuart