riscv: Add ptrace vector support

This patch adds ptrace support for riscv vector. The vector registers will
be saved in datap pointer of __riscv_v_ext_state. This pointer will be set
right after the __riscv_v_ext_state data structure then it will be put in
ubuf for ptrace system call to get or set. It will check if the datap got
from ubuf is set to the correct address or not when the ptrace system call
is trying to set the vector registers.

Co-developed-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>