selftests: riscv: Add test for the Vector ptrace interface

selftests: riscv: Add test for the Vector ptrace interface

Add a test case that does some basic verification of the Vector ptrace
interface. This forks a child process then using ptrace to inspect and
manipulate the v31 register of the child.

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Andy Chiu <andybnac@gmail.com>
Tested-by: Andy Chiu <andybnac@gmail.com>
Link: https://patch.msgid.link/20251013091318.467864-3-yongxuan.wang@sifive.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>