CJALR unrepresentability

[PeterRugg]

As reported by Dapeng Gao:

In the Sail specification of CJALR, the link address is computed using setAddr, but in Flute, it is done by incrementing the offset of rg_pcc, which involves the fast representability check.

[PeterRugg]

I would have suspected that the link address would always have to be in bounds, since otherwise the instruction doesn't link. In-bounds capabilities shouldn't have a risk of being unrepresentable, I believe (even adding +4 to move onto the next instruction). If any of these assumptions aren't true then that would be concerning!