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/arch/x86_64/gdt_asm.S

@@ -6,13 +6,13 @@
 gdt_reload:
     lgdt [rip + gdt_pointer]
 
-    push 8
+    push 0x08
     lea rax, [rip + .flush]
     push rax
     lretq
 
 .flush:
-    mov eax, 0x10
+    mov ax, 0x10
     mov ds, ax
     mov es, ax
     mov fs, ax