1.0.42.33: ppc: Improve pseudo-atomic implementation

1.0.42.33: ppc: Improve pseudo-atomic implementation

  * The runtime (ppc-assem.S) used a shorter, more concise
    sequence of instructions for testing pseudo-atomic than the
    compiler (ppc/macros.lisp) did.  Altered the compiler to use
    the same sequence as the runtime.

  * Both the compiler and the runtime used magic numbers in the
    pseudo-atomic sequence instead of using named constants.
    Fixed, using the existing named constants.