Add ARM CPU feature detection support for OpenBSD.

Index: vpx_ports/aarch32_cpudetect.c
--- vpx_ports/aarch32_cpudetect.c.orig
+++ vpx_ports/aarch32_cpudetect.c
@@ -57,7 +57,7 @@ static int arm_get_cpu_caps(void) {
   return flags;
 }
 
-#elif defined(__linux__)  // end defined(VPX_USE_ANDROID_CPU_FEATURES)
+#elif defined(__linux__) || defined(__OpenBSD__)  // end defined(VPX_USE_ANDROID_CPU_FEATURES)
 
 #include <sys/auxv.h>
 
@@ -67,7 +67,12 @@ 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 || HAVE_NEON_ASM
   if (hwcap & VPX_AARCH32_HWCAP_NEON) {
     flags |= HAS_NEON;
