m1x/x86_64: Add GDT related definitions

Signed-off-by: Ian Moffett <ian@mirocom.org>

src/m1x/include/arch/x86_64/gdt.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2026, Mirocom Laboratories
+ * Provided under the BSD-3 clause
+ */
+
+#ifndef _MACHINE_GDT_H_
+#define _MACHINE_GDT_H_ 1
+
+/* Kernel code/data */
+#define GDT_KCODE  0x08
+#define GDT_KDATA  0x10
+
+/* User code/data */
+#define GDT_UCODE  0x18
+#define GDT_UDATA  0x20
+
+/* Task state segment */
+#define GDT_TSS    0x28
+#define GDT_TSS_INDEX 0x05
+
+#endif  /* !_MACHINE_GDT_H_ */