fix(arch/x86_64): 重构信号栈帧结构以兼容Linux

[fslongjin]

• 新增与Linux完全兼容的ucontext、sigcontext、fpstate结构体
• 实现内核与用户态FP状态的安全转换
• 改进信号处理函数的寄存器恢复逻辑
• 增强SROP攻击防护机制
• 修复信号处理中FP状态保存与恢复的问题

[Copilot]

Pull Request Overview

This PR refactors the x86_64 signal stack frame structure to achieve full Linux compatibility. It replaces the previous custom signal handling implementation with Linux-compatible structures for ucontext, sigcontext, and fpstate, enabling safer conversion of FP state between kernel and userspace while improving SROP attack protection.

Key changes:

• Introduced Linux-compatible signal structures (UserUContext, UserSigContext, UserFpState64) with compile-time offset verification
• Implemented bidirectional FP state conversion with FXSAVE format handling
• Enhanced SROP protection by validating fpstate pointer integrity during restoration

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
user/apps/tests/syscall/gvisor/whitelist.txt	Added new test case fpsig_mut_test to the test whitelist
kernel/src/arch/x86_64/ipc/signal.rs	Complete refactoring of signal handling: new Linux-compatible structures, FP state conversion methods, improved sys_rt_sigreturn and setup_frame functions with better error handling and SROP protection

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