powerpc: sstep: Add support for darn instruction

This adds emulation support for the following integer instructions:
  * Deliver A Random Number (darn)

As suggested by Michael, this uses a raw .long for specifying the
instruction word when using inline assembly to retain compatibility
with older binutils.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>