param.h : enable defines for PPC970 on DarwinOS

fixes:
gemm.c: In function 'sgemm_':
../common_param.h:981:18: error: 'SGEMM_DEFAULT_P' undeclared (first use in this function)
 #define SGEMM_P  SGEMM_DEFAULT_P
                  ^

param.h

@@ -1999,7 +1999,7 @@ USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #define ZGEMM_DEFAULT_UNROLL_M 2
 #define ZGEMM_DEFAULT_UNROLL_N 2
 
-#ifdef OS_LINUX
+#if defined(OS_LINUX) || defined(OS_DARWIN)
 #if L2_SIZE == 1024976
 #define SGEMM_DEFAULT_P 320
 #define DGEMM_DEFAULT_P 256