[aarch64-port-dev ] 8u-aarch64: Backport 8247979: aarch64: missing side effect of killing flags for clearArray_reg_reg

[Yangfei (Felix)]

Hi,

Original Bug: https://bugs.openjdk.java.net/browse/JDK-8247979 
Original patch: https://hg.openjdk.java.net/jdk/jdk/rev/9fce19fdda7e 
Review thread: https://mail.openjdk.java.net/pipermail/aarch64-port-dev/2020-June/009120.html 

As mentioned in the review thread, this poses similar risk as bug: 8224828: aarch64: rflags is not correct after safepoint poll. 
It will be very hard to find out if this kind of bug triggers. 

Webrev for 8u-aarch64: http://cr.openjdk.java.net/~fyang/8247979-8u/webrev.00/ 
This handles both clearArray_reg_reg and clearArray_imm_reg as rflags can be clobbered by both of them. 
This is not the case for clearArray_imm_reg in jdk11+ versions.

Performed full jtreg test with 8u aarch64 release build.  OK to backport?

Thanks,
Felix