[RISCV] Set ShouldSignExtI32Param in TargetLibraryInfo for riscv64.

riscv64 sign extends signed and unsigned i32 arguments like Mips.

Based on discussion here
https://discourse.llvm.org/t/can-we-preserve-signext-return-attribute-when-converting-memcmp-to-bcmp/67126

I'll work on returns next.

Differential Revision: https://reviews.llvm.org/D139893