sys: Refactor GDT initialization, enhance syscall handling, and improve user mode entry

Finally we got usermode working again, It took awhile but we got it done. A few things we did in this commit:

- Simplified GDT entry initialization in gdt.c for kernel and user segments.
- Fixed TSS structure for task switching.
- Implemented cr2 handling in a page fault.
- Enhanced user stack setup in usermode.c to return the correct RSP.
- Improved syscall implementation in syscall.c, including new syscall numbers.
- Updated syscall entry to correctly handle context switching and argument passing.
- Refactored init.c to demonstrate file operations using syscalls.
- Added new syscalls for file operations in syscalls.h.
- Modified VFS to handle leading slashes in paths correctly.

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/syscalls.h

@@ -1,5 +1,10 @@
 #pragma once
 
+#define SYS_READ   0
+#define SYS_WRITE  1
+#define SYS_OPEN   2
+#define SYS_CLOSE  3
+
 static inline long syscall(long num, long a1, long a2, long a3)
 {
     long ret;
@@ -8,7 +13,6 @@ static inline long syscall(long num, long a1, long a2, long a3)
         "syscall"
         : "=a"(ret)
         : "a"(num), "D"(a1), "S"(a2), "d"(a3)
-        :
     );
 
     return ret;