Add AArch64 CPU feature detection support for OpenBSD.

Index: aom_ports/aarch64_cpudetect.c
--- aom_ports/aarch64_cpudetect.c.orig
+++ aom_ports/aarch64_cpudetect.c
@@ -127,7 +127,7 @@ 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>
 
@@ -142,10 +142,20 @@ static int arm_get_cpu_caps(void) {
 static int arm_get_cpu_caps(void) {
   int flags = 0;
 #if HAVE_ARM_CRC32 || HAVE_NEON_DOTPROD || HAVE_SVE
+#ifdef __OpenBSD__
+  unsigned long hwcap = 0;
+  elf_aux_info(AT_HWCAP, &hwcap, sizeof(hwcap));
+#else
   unsigned long hwcap = getauxval(AT_HWCAP);
 #endif
+#endif
 #if HAVE_NEON_I8MM || HAVE_SVE2
+#ifdef __OpenBSD__
+  unsigned long hwcap2 = 0;
+  elf_aux_info(AT_HWCAP2, &hwcap2, sizeof(hwcap2));
+#else
   unsigned long hwcap2 = getauxval(AT_HWCAP2);
+#endif
 #endif
 
 #if HAVE_NEON
