riscv64: use vdso for __riscv_hwprobe().

Also de-pessimize time(), where the vdso entrypoint only exists on
x86/x86-64 anyway.

Bug: https://github.com/google/android-riscv64/issues/8
Test: strace
Change-Id: I14cb2a3130b6ff88d06d43ea13d3a825a26de290
diff --git a/libc/private/bionic_asm_offsets.h b/libc/private/bionic_asm_offsets.h
index c2f2b56..e72adda 100644
--- a/libc/private/bionic_asm_offsets.h
+++ b/libc/private/bionic_asm_offsets.h
@@ -28,6 +28,6 @@
 
 #pragma once
 
-#ifdef __aarch64__
-#define OFFSETOF_libc_globals_memtag_stack 80
+#if defined(__aarch64__)
+#define OFFSETOF_libc_globals_memtag_stack 64
 #endif