RFR(S)[13]: AArch64: float point register corruption in ZBarrierSetAssembler::load_at

[Stuart Monteith]

Hello,
   There are incorrect values on VarHandle JCStress tests when ZGC is
enabled. A line was omitted that should have saved the first first
four registers. This change also removes a dependency on rscratch1 and
reuses rscratch2.

Bug: https://bugs.openjdk.java.net/browse/JDK-8226515
Webrev: http://cr.openjdk.java.net/~smonteith/8226515/webrev.0/

Tested with JCStress, jtreg tier1 and SpecJBB2015.

This should be applied to jdk/jdk13 and jdk/jdk

Thanks.