Riscv: Enable vector extensions for QEMU

Test: step over vsetvli in __dl_memset_v
Change-Id: Ie201f1ed543b147884f5577e82ec545e72d5207e
diff --git a/tools/buildbot-vm.sh b/tools/buildbot-vm.sh
index fef2967..751461e 100755
--- a/tools/buildbot-vm.sh
+++ b/tools/buildbot-vm.sh
@@ -111,6 +111,7 @@
             -nographic \
             -bios fw_jump.elf \
             -kernel uboot.elf \
+            -cpu rv64,v=true,vlen=256,vext_spec=v1.0 \
             -drive file="$ART_TEST_VM_IMG",if=virtio \
             -drive file=user-data.img,format=raw,if=virtio \
             -device virtio-net-device,netdev=usernet \