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/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 079acf9..f4f9e1b 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -357,7 +357,7 @@
 
 # riscv64-specific
 int __riscv_flush_icache:riscv_flush_icache(void*, void*, unsigned long) riscv64
-int __riscv_hwprobe:riscv_hwprobe(riscv_hwprobe*, size_t, size_t, unsigned long*, unsigned) riscv64
+int riscv_hwprobe(riscv_hwprobe*, size_t, size_t, unsigned long*, unsigned) riscv64
 
 # x86-specific
 int     __set_thread_area:set_thread_area(void*) x86