Add support for runtime CPU feature check on POWER on FreeBSD.

1. Code uses PPC_FEATURE_HAS_VSX, but it's not checked similarly to
PPC_FEATURE2_ARCH_3_00 and PPC_FEATURE2_ARCH_3_00 for availability. FreeBSD has
those macros in machine/cpu.h, but I went with the way chosen for
PPC_FEATURE2_ARCH_3_00 and PPC_FEATURE2_ARCH_3_00. Other than that, FreeBSD also
has sys/auxv.h and that's where elf_aux_info() is defined.
2. getauxval() is actually Linux-only, but code checked for __unix__. It won't
work on all UNIX, so change it back to __linux__. Add another code variant
strictly for FreeBSD.
3. Update comment. This commit adds code for FreeBSD, but recently there
appeared support for powerpc64 in OpenBSD.

modules/core/src/system.cpp

@@ -155,6 +155,9 @@ void* allocSingletonNewBuffer(size_t size) { return malloc(size); }
 # ifndef PPC_FEATURE2_ARCH_3_00
 #   define PPC_FEATURE2_ARCH_3_00 0x00800000
 # endif
+# ifndef PPC_FEATURE_HAS_VSX
+#   define PPC_FEATURE_HAS_VSX 0x00000080
+# endif
 #endif
 
 #if defined _WIN32 || defined WINCE
@@ -607,7 +610,7 @@ struct HWFeatures
         have[CV_CPU_MSA] = true;
     #endif
 
-    #if (defined __ppc64__ || defined __PPC64__) && defined __unix__
+    #if (defined __ppc64__ || defined __PPC64__) && defined __linux__
         unsigned int hwcap = getauxval(AT_HWCAP);
         if (hwcap & PPC_FEATURE_HAS_VSX) {
             hwcap = getauxval(AT_HWCAP2);
@@ -617,8 +620,19 @@ struct HWFeatures
                 have[CV_CPU_VSX] = (hwcap & PPC_FEATURE2_ARCH_2_07) != 0;
             }
         }
+    #elif (defined __ppc64__ || defined __PPC64__) && defined __FreeBSD__
+        unsigned int hwcap = 0;
+        elf_aux_info(AT_HWCAP, &hwcap, sizeof(hwcap));
+        if (hwcap & PPC_FEATURE_HAS_VSX) {
+            elf_aux_info(AT_HWCAP2, &hwcap, sizeof(hwcap));
+            if (hwcap & PPC_FEATURE2_ARCH_3_00) {
+                have[CV_CPU_VSX] = have[CV_CPU_VSX3] = true;
+            } else {
+                have[CV_CPU_VSX] = (hwcap & PPC_FEATURE2_ARCH_2_07) != 0;
+            }
+        }
     #else
-        // TODO: AIX, FreeBSD
+        // TODO: AIX, OpenBSD
         #if CV_VSX || defined _ARCH_PWR8 || defined __POWER9_VECTOR__
             have[CV_CPU_VSX] = true;
         #endif