The MLAS POWER kernels are VSX code, and inc/mlas.h only defines
MLAS_TARGET_POWER when __VSX__ is set. The CMake side instead keys off
CMAKE_SYSTEM_PROCESSOR, so pre-VSX 64-bit PowerPC targets (POWER4/POWER5,
970/G5, Cell) compile POWER kernels that platform.cpp never dispatches to,
while the generic code path is left without its MlasSgemmKernelZero/Add
implementation.

Probe __VSX__ the same way the sources do and fall back to the scalar
kernels when the target has no VSX.

--- opencv-5.0.0.vanilla/3rdparty/mlas/CMakeLists.txt
+++ opencv-5.0.0/3rdparty/mlas/CMakeLists.txt
@@ -156,6 +156,21 @@ endif()
 # POWER (ppc64le / AIX). Always compile the base SgemmKernelPower; opt-in
 # POWER10 if the compiler supports -mcpu=power10.
 if(MLAS_POWER)
+  # The POWER kernels are VSX code, and inc/mlas.h only defines
+  # MLAS_TARGET_POWER when __VSX__ is set. Pre-VSX 64-bit PowerPC targets
+  # (POWER4/POWER5, 970/G5, Cell) match the CMAKE_SYSTEM_PROCESSOR test above
+  # but build kernels platform.cpp never dispatches to, so key the kernel
+  # choice off the same __VSX__ the sources use.
+  include(CheckCXXSourceCompiles)
+  check_cxx_source_compiles("
+    #if !defined(__VSX__)
+    #error target has no VSX
+    #endif
+    int main() { return 0; }" MLAS_HAS_VSX)
+  set(MLAS_HAS_VSX ${MLAS_HAS_VSX} CACHE INTERNAL "" FORCE)
+endif()
+
+if(MLAS_POWER AND MLAS_HAS_VSX)
   list(APPEND mlas_platform_srcs ${MLAS_SRC_DIR}/power/SgemmKernelPower.cpp)
   set_source_files_properties(${MLAS_SRC_DIR}/power/SgemmKernelPower.cpp
     PROPERTIES COMPILE_FLAGS "-DSINGLE")
@@ -172,6 +187,10 @@ if(MLAS_POWER)
     set_source_files_properties(${MLAS_SRC_DIR}/power/SgemmKernelPOWER10.cpp
       PROPERTIES COMPILE_FLAGS "-O2 -mcpu=power10 -DSINGLE")
   endif()
+elseif(MLAS_POWER)
+  message(STATUS "MLAS: ${CMAKE_SYSTEM_PROCESSOR} target has no VSX; "
+                 "using the scalar SGEMM kernels")
+  list(APPEND mlas_platform_srcs ${MLAS_SRC_DIR}/scalar/SgemmKernelScalar.cpp)
 endif()
 
 # LoongArch 64 (LSX + LASX).