Faster iq4_xs_r4 on Zen4

The trick is to simply prepare the Q8 block sums for
blocks of 32 as floats. This brings PP-512 up to 254.6 t/s
from 224 t/s.

ggml/include/ggml.h

@@ -407,6 +407,7 @@ extern "C" {
         GGML_TYPE_IQ2_KS  = 145,
         GGML_TYPE_IQ4_KSS = 146,
         GGML_TYPE_Q8_K16  = 147,
+        GGML_TYPE_Q8_K32  = 148,
 
         GGML_TYPE_Q4_0_R4   = 202,
         GGML_TYPE_Q5_0_R4   = 206,