ggml-cpu: Enable tiled matmul on AIX (ggml-org#25199)

The matmul_tiled path uses large local stack buffers for A_pack and B_pack. On AIX this can trigger a segmentation fault, so reduce the buffer footprint there to keep the tiled path usable.

 Performance Impact:
    ~ 2x gains in PP_Speed for FP32, Q4_0 and Q8_0 models tested with llama-bench, llama-batched-bench and llama-cli.
    Models used: Llama3.2 3b Instruct F32, qwen 2.5 3b Q4_0 and Q8_0

Author: shalinib-ibm

ggml/src/ggml-cpu/llamafile/sgemm.cpp

@@ -2321,24 +2321,28 @@ class tinyBLAS_Q0_PPC {
     }
 
     void matmul(int64_t m, int64_t n) {
-    #if defined(_AIX) || defined(__BIG_ENDIAN__)
-        mnpack(0, m, 0, n);
-    #else
-        const int64_t mc = 64;
-        const int64_t kc = 64;
+        int64_t mc = 64;
         int64_t nc = 64;
+        int64_t kc = 64;
+        int64_t n_chunk = 64;
+    #if defined(_AIX) || defined(__BIG_ENDIAN__)
+        mc = 32;
+        nc = 32;
+        kc = 32;
+        n_chunk = 32
+    #endif
         int64_t n_aligned = 0;
-        if (n % 64 == 0) {
+        if (n % n_chunk == 0) {
             n_aligned = n;
         } else if (n == 4) {
             n_aligned = 4;
-        } else if (n < 64) {
+        } else if (n < n_chunk) {
             n_aligned = (n / 8) * 8;
         } else {
-            n_aligned = (n / 64) * 64;
+            n_aligned = (n / n_chunk) * n_chunk;
         }
         if (n_aligned > 0) {
-            if (n_aligned % 64 == 0)      nc = 64;
+            if (n_aligned % n_chunk == 0) nc = n_chunk;
             else if (n_aligned == n)      nc = n;
             else if (n_aligned % 32 == 0) nc = 32;
             else if (n_aligned % 24 == 0) nc = 24;
@@ -2354,7 +2358,6 @@ class tinyBLAS_Q0_PPC {
         } else {
             mnpack(0, m, 0, n);
         }
-    #endif
     }
 
   private:
@@ -3195,16 +3198,19 @@ class tinyBLAS_PPC {
     }
 
     void matmul(int64_t m, int64_t n) {
+        int64_t mc = 256;
+        int64_t nc = 256;
+        int64_t kc = 256;
     #if defined(_AIX) || defined(__BIG_ENDIAN__)
-        mnpack(0, m, 0, n);
-    #else
-        int64_t mc = 256; int64_t nc = 256; int64_t kc = 256;
+        mc = 128;
+        nc = 128;
+        kc = 128;
+    #endif
         if (m % mc == 0 && n % nc == 0 && k % kc == 0) {
             matmul_tiled(m, n, mc, nc, kc);
         } else {
             mnpack(0, m, 0, n);
         }
-    #endif
     }
 
   private: