Add ARM CPU feature detection support for OpenBSD.

Index: aom_ports/aarch32_cpudetect.c
--- aom_ports/aarch32_cpudetect.c.orig
+++ aom_ports/aarch32_cpudetect.c
@@ -55,7 +55,7 @@ static int arm_get_cpu_caps(void) {
   return flags;
 }
 
-#elif defined(__linux__)  // end defined(AOM_USE_ANDROID_CPU_FEATURES)
+#elif defined(__linux__) || defined(__OpenBSD__)  // end defined(AOM_USE_ANDROID_CPU_FEATURES)
 
 #include <sys/auxv.h>
 
@@ -65,7 +65,12 @@ static int arm_get_cpu_caps(void) {
 
 static int arm_get_cpu_caps(void) {
   int flags = 0;
+#ifdef __OpenBSD__
+  unsigned long hwcap = 0;
+  elf_aux_info(AT_HWCAP, &hwcap, sizeof(hwcap));
+#else
   unsigned long hwcap = getauxval(AT_HWCAP);
+#endif
 #if HAVE_NEON
   if (hwcap & AOM_AARCH32_HWCAP_NEON) flags |= HAS_NEON;
 #endif  // HAVE_NEON
