RFR(M): 8204240: Extend MDO to allow more reasons to be recorded per bci

[Roland Westrelin]

> Note this patch modifies platform specific code. I verified x86 (32 and
> 64 bits), aarch64. Sparc is ok AFAICT but I couldn't verify it. For PPC
> and arm32 I will need someone to take a closer look.

Both Andrews took a look at aarch64. Martin confirmed PPC64 and s390 are
ok. I looked at the arm32 code and I think no change is needed.

I now need another review so I can push this. Nils, do you still intend
to review it?

Roland.