[RISCV] Use unsigned instead of signed types for Zk* and Zb* builtins.

Unsigned is a better representation for bitmanipulation and cryptography.w

The only exception being the return values for clz and ctz intrinsics is
a signed int. That matches the target independent clz and ctz builtins.

This is consistent with the current scalar crypto proposal
https://github.com/riscv-non-isa/riscv-c-api-doc/pull/44

Reviewed By: VincentWu

Differential Revision: https://reviews.llvm.org/D154616