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/idt.c

@@ -0,0 +1,46 @@
+#include "idt.h"
+#include <stdint.h>
+#include <stddef.h> /* for memset */
+
+idt_entry_t idt_entries[256];
+idt_ptr_t   idt_ptr;
+
+extern void idt_load();
+
+static void idt_set_gate(uint8_t num, uint32_t base, uint16_t sel, uint8_t flags)
+{
+    idt_entries[num].base_lo = base & 0xFFFF;
+    idt_entries[num].base_hi = (base >> 16) & 0xFFFF;
+
+    idt_entries[num].sel     = sel;
+    idt_entries[num].always0 = 0;
+    // We must uncomment the OR below when we get to using user-mode.
+    // It sets the interrupt gate's privilege level to 3.
+    idt_entries[num].flags   = flags /* | 0x60 */;
+}
+
+/* Note: memset implementation because we don't link with libc */
+static void *memset(void *s, int c, size_t n)
+{
+    unsigned char *p = s;
+    while(n--)
+        *p++ = (unsigned char)c;
+    return s;
+}
+
+void init_idt()
+{
+    idt_ptr.limit = sizeof(idt_entry_t) * 256 - 1;
+    idt_ptr.base  = (uint32_t)&idt_entries;
+
+    memset(&idt_entries, 0, sizeof(idt_entry_t) * 256);
+
+    /* Determine valid flags:
+     * 0x8E = 1000 1110
+     * P=1, DPL=00, S=0 (System), Type=1110 (32-bit Interrupt Gate) */
+
+    /* For now, just load the IDT without any entries to verify loading works without crashing.
+       Later we will set up ISRs. */
+
+    idt_load((uint32_t)&idt_ptr);
+}