[RISCV] Exclude X1 and X5 from register scavenging for long branch. (#80215)

When a branch target is too far away we need to emit an indirect branch.
We scavenge a register for this since we don't know we need this until
after register allocation.

Jumps using X1 and X5 as the source are hints to the hardware to pop the
return-address stack. We should avoiding using them for jumps that
aren't a return or tail call.