feat: Added syscalls and began on usermode support, still no scheduler

kernel/src/arch/gdt-helper.asm

@@ -0,0 +1,24 @@
+global flush_tss
+global jump_user
+extern test_user
+
+flush_tss:
+    mov ax, 5 * 8
+    ltr ax
+    ret
+
+; void jump_user(uint64_t addr, uint64_t stack)
+jump_user:
+    mov ax, 0x23  ; Ring 3 data with bottom 2 bits set for ring 3
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax   ; SS is handled by iretq
+
+    ; Set up the stack frame iretq expects
+    push 0x23     ; Data selector
+    push rsi      ; Stack
+    pushf         ; Rflags
+    push 0x1B     ; Code selector (ring 3 code with bottom 2 bits set for ring 3)
+    push rdi      ; Instruction address to return to
+    iretq