powerpc/sstep: Add emulation support for ‘setb’ instruction

This adds emulation support for the following instruction:
   * Set Boolean (setb)

Signed-off-by: Sathvika Vasireddy <sathvika@linux.vnet.ibm.com>
Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7b735b0c898da0db2af8628a64df2f5114596f22.1620727160.git.sathvika@linux.vnet.ibm.com