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>

src/syscall/syscall.h

@@ -1,3 +1,10 @@
 #pragma once
 
+
+#define SYS_READ    0
+#define SYS_WRITE   1
+#define SYS_OPEN    2
+#define SYS_CLOSE   3
+
+
 void syscall_init(void);