[8u] RFR: fix of vtable self-patching routine

[Fedor Burdun]

Hi all,

I would like to ask you several reviews of proposed fix of issue below.

The problem is crash caused by using CDS.
It occurs because c++ vtables self-patching routine (written in metaspaceShared_aarch32.cpp) corrupts r9 register, that can be used by c++ compiler to store locals.
Usually it happens in Method::restore_vtable virtual call, that causes undefined behaviour (mostly crahes) in InstanceKlass::restore_unshareable_info function.

Reproduction steps:
# dump shared space
  $ java -Xint -XX:+UnlockDiagnosticVMOptions -XX:SharedArchiveFile=./sample.jsa -Xshare:dump

# restore from CDS (crash happens here)
  $ java -Xint -XX:+UnlockDiagnosticVMOptions -XX:SharedArchiveFile=./sample.jsa -Xshare:on -version

Fix is in attachement.

Thanks,
Fedor
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