Full generated stub save and restore PR now in draft

[Andrew Dinn]

I have a draft PR against mainline which implements save and restore for 
all generated stubs:

   https://github.com/openjdk/jdk/pull/28433

It appears to be functioning correctly on both aarch64 and x86 but a run 
of the full Oracle test suite would be helpful. The patch switches on 
stub save and restore by default and I have updated test AOTCodeFlags so 
it exercises stub save and restore with and without adapter save and 
restore and this passes on both arches.

This PR not only saves and restores code it also sorts out a lot of the 
mess of publishing and updating external and stub addresses as a side 
effect of loading or generating stubs. This is an important step for us 
to be able to make nmethod save and restore reliable when we add it 
(which partly explains why I targeted this to mainline).

regards,


Andrew Dinn
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