[RISCV][MC] Implicit 0-offset aliases for JR/JALR (#94688)

This broadly follows how in almost all places, we accept `(<reg>)` to
mean `0(<reg>)`, but I think these are the first like this for Jumps
rather than Loads/Stores. These are accepted by binutils but not by
LLVM: https://godbolt.org/z/GK7MGE7q7