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_flush.S

@@ -0,0 +1,18 @@
+.section .text
+.global gdt_flush
+.type gdt_flush, @function
+gdt_flush:
+    mov 4(%esp), %eax
+    lgdt (%eax)
+    
+    mov $0x10, %ax
+    mov %ax, %ds
+    mov %ax, %es
+    mov %ax, %fs
+    mov %ax, %gs
+    mov %ax, %ss
+    
+    ljmp $0x08, $flush
+
+flush:
+    ret