Reserve relocations for the RISC-V Y base ISA.

[resistor]

Per the defined criteria for reserving relocation numbers ahead of standardization, we would like to reserve 10 relocation numbers for the RISCV Y base ISA. Our current best estimate (based on existing CHERI ABI implementations and outstanding Y specification work items) is that the Y ISA will require 7 relocations. We prefer to round that up to 10 relocations for this reservation to reduce the risk of conflicts, with the intention of releasing any reserved relocation numbers that are unused once the Y base ABI is standardized.

[resistor]

Tagging @jrtc27 as RISCV Y interested party.

[jrtc27]

It occurs to me that #452 also tries to allocate 66 (and should now also really be reserving it until the ABI is ready), so we should probably be good citizens and not make Kito and other CFI people unhappy. I don't immediately recall if there are other outstanding PRs that allocate relocations that we should care about (i.e. aren't abandoned / stalled) and would have reserved encodings already had that policy existed when they were conceived. So perhaps worth holding off for a few days to see if @kito-cheng or other psABI-interested parties remember any others before updating this PR to use 67-76?

[lenary]

Just going through the still-open PRs:

• Add R_RISCV_ALIGN_DOWN #392 defines another relocation, but a) it conflicts with a TLSDESC reloc (also at 62), so will need to be moved anyway, and b) the proposal seems to have stalled for a good long while.
• FDPIC (also seems stalled) seems to define new relocs but no specific codes for them.

I didn't see any others trying to reserve relocations, and I don't remember more than that.

[lenary]

I am happy with this.

[kito-cheng]

LGTM, thanks for reserving 66 for landing pad, I probably should create another PR after this PR merge to update the text a little bit.

For #392...that's just inactive for long time so it's OK to skip that IMO :P

Will merge once @jrtc27 also approve since this is kinda doc change only not require any PoC or approve from any open source place holder.