lib: Support vector extension

Enable vector context in mstatus by updating the corresponding bits
in mstatus if vector extension is supported by the hart.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>