Add AArch64 CPU feature detection support for OpenBSD.

Index: vpx_ports/aarch64_cpudetect.c
--- vpx_ports/aarch64_cpudetect.c.orig
+++ vpx_ports/aarch64_cpudetect.c
@@ -120,7 +120,7 @@ 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>
 
@@ -134,10 +134,20 @@ static int arm_get_cpu_caps(void) {
 static int arm_get_cpu_caps(void) {
   int flags = 0;
 #if 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  // HAVE_NEON_DOTPROD || HAVE_SVE
 #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  // HAVE_NEON_I8MM || HAVE_SVE2
 #if HAVE_NEON
   flags |= HAS_NEON;  // Neon is mandatory in Armv8.0-A.
