cmake : fix compile assumptions for power9/etc (whisper/2777)

* Add small comment re: VSX to readme

Co-authored-by: midnight <midnight@example.com>

ggml/src/ggml-cpu/CMakeLists.txt

@@ -281,19 +281,15 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
         endif()
     elseif (${CMAKE_SYSTEM_PROCESSOR} MATCHES "ppc64")
         message(STATUS "PowerPC detected")
-        execute_process(COMMAND bash -c "grep POWER10 /proc/cpuinfo | head -n 1" OUTPUT_VARIABLE POWER10_M)
-        string(FIND "${POWER10_M}" "POWER10" substring_index)
-        if (NOT DEFINED substring_index OR "${substring_index}" STREQUAL "")
-            set(substring_index -1)
-        endif()
-
-        if (${substring_index} GREATER_EQUAL 0)
-        list(APPEND ARCH_FLAGS -mcpu=power10)
+        execute_process(COMMAND bash -c "grep POWER /proc/cpuinfo | head -n 1" OUTPUT_VARIABLE POWER_M)
+        if (${POWER_M} MATCHES "POWER10")
+            list(APPEND ARCH_FLAGS -mcpu=power10)
+        elseif (${POWER_M} MATCHES "POWER9")
+            list(APPEND ARCH_FLAGS -mcpu=power9)
         elseif (${CMAKE_SYSTEM_PROCESSOR} MATCHES "ppc64le")
-        list(APPEND ARCH_FLAGS -mcpu=powerpc64le)
+            list(APPEND ARCH_FLAGS -mcpu=powerpc64le -mtune=native)
         else()
-            list(APPEND ARCH_FLAGS -mcpu=native -mtune=native)
-            # TODO: Add  targets for Power8/Power9 (Altivec/VSX) and Power10(MMA) and query for big endian systems (ppc64/le/be)
+            list(APPEND ARCH_FLAGS -mcpu=powerpc64 -mtune=native)
         endif()
     elseif (${CMAKE_SYSTEM_PROCESSOR} MATCHES "loongarch64")
         message(STATUS "loongarch64 detected")