[RISCV] Split clmul/clmulh/clmulr builtins into _32 and _64 versions.

This removes another use of 'long' to mean xlen from builtins.

I've also converted the types to unsigned as proposed in D154616.

clmul_32 is available to RV64 as its emulation is clmul+sext.w
clmulh_32 and clmulr_32 are not available on RV64 as their emulation
is currently 6 instructions in the worst case.