powerpc/32s: Fix LLVM SMP build

LLVM assembler does not recognise 3-operand cmpi, use cmpwi.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606131828.315427-1-npiggin@gmail.com