[RISCV] Generate 32 bits jumptable entries when code model is small

The code can only address the whole RV32 address space or the lower 2 GiB
of the RV64 address space in small code model, so 32 bits entry is enough.
Cache hit ratio and code size have some improvements.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D116435