Implement GDT and basic IDT setup

This commit adds GDT initialization with proper code/data segments and reloads CS.

It also adds the initial IDT structure and loads an empty IDT.

Build configuration updated to disable SSE/MMX to prevent compiler generation of unsupported instructions in early boot.

src/gdt.h

@@ -0,0 +1,25 @@
+#ifndef GDT_H
+#define GDT_H
+
+#include <stdint.h>
+
+/* GDT Entry Structure */
+struct gdt_entry {
+    uint16_t limit_low;
+    uint16_t base_low;
+    uint8_t  base_middle;
+    uint8_t  access;
+    uint8_t  granularity;
+    uint8_t  base_high;
+} __attribute__((packed));
+
+/* GDT Pointer Structure */
+struct gdt_ptr {
+    uint16_t limit;
+    uint32_t base;
+} __attribute__((packed, aligned(1)));
+
+/* Initialize GDT */
+void init_gdt(void);
+
+#endif // GDT_H