[Backport] riscv control-flow integrity for usermode

[zhuzhenxxx-collab]

Backporting the riscv control-flow integrity for usermode patch series v23 to this tree.

This series adds RISC-V user-mode CFI support, including:

• zicfilp / zicfiss enumeration and runtime support
• user shadow stack task state and CSR save/restore
• prctl interfaces for shadow stack and indirect branch tracking
• signal, fork/clone, ptrace, hwprobe, and vDSO changes
• selftests and documentation for the new CFI support