CVE-2025-38435 riscv: vector: Fix context save/restore with xtheadvector

In the Linux kernel, the following vulnerability has been resolved: riscv: vector: Fix context save/restore with xtheadvector Previously only v0-v7 were correctly saved/restored, and the context of v8-v31 are damanged. Correctly save/restore v8-v31 to avoid breaking userspace...