Add RISC-V CPU feature detection support for OpenBSD.

Index: aom_ports/riscv_cpudetect.c
--- aom_ports/riscv_cpudetect.c.orig
+++ aom_ports/riscv_cpudetect.c
@@ -26,7 +26,13 @@
 int riscv_simd_caps(void) {
   int flags = 0;
 #if HAVE_RVV
+#ifdef __OpenBSD__
+  unsigned long hwcap;
+  if (elf_aux_info(AT_HWCAP, &hwcap, sizeof(hwcap)) != 0)
+	hwcap = 0;
+#else
   unsigned long hwcap = getauxval(AT_HWCAP);
+#endif
   if (hwcap & HWCAP_RVV) flags |= HAS_RVV;
 #endif
   return flags;
