Kernel: Fix retreiving frame pointer from a thread

If we're trying to walk the stack for another thread, we can
no longer retreive the EBP register from Thread::m_tss. Instead,
we need to look at the top of the kernel stack, because all threads
not currently running were last in kernel mode. Context switches
now always trigger a brief switch to kernel mode, and Thread::m_tss
only is used to save ESP and EIP.

Fixes #2678

Kernel/Arch/i386/CPU.h

@@ -775,6 +775,7 @@ public:
     void switch_context(Thread* from_thread, Thread* to_thread);
     [[noreturn]] static void assume_context(Thread& thread, u32 flags);
     u32 init_context(Thread& thread, bool leave_crit);
+    static bool get_context_frame_ptr(Thread& thread, u32& frame_ptr, u32& eip);
     
     void set_thread_specific(u8* data, size_t len);
 };