arch/x86: Unify GDT entries

Currently there are 3 GDTs (Global Descriptor Tables) being used on x86:
- preRAM (gdt_init.S)
- SMM (smm_stub.S)
- RAM (c_start.S)

They have different layouts and thus different offsets for the segments
being used in assembly code. Stop using different GDT segments and
ensure that for ROM (preRAM + SMM) and RAM (ramstage) the segments
match. RAM will have additional entries, not found in pre RAM GDT,
but the segments for protected mode and 64-bit mode now match in
all stages.

This allows to use the same defines in all stages. It also drops the
need to know in which stage the code is compiled and it's no longer
necessary to switch the code segment between stages.

While at it fix the comments in the ramstage GDT and drop unused
declarations from header files, always set the accessed bit and drop
GDT_CODE_ACPI_SEG.

Change-Id: I208496e6e4cc82833636f4f42503b44b0d702b9e
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87255
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Shuo Liu <shuo.liu@intel.com>
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>

src/include/cpu/x86/gdt.h

@@ -3,16 +3,28 @@
 #ifndef CPU_X86_GDT
 #define CPU_X86_GDT
 
+#ifndef __ASSEMBLER__
 /* These symbols are defined in c_start.S. */
 extern char gdt[];
 extern char per_cpu_segment_descriptors[];
 extern uint32_t per_cpu_segment_selector;
 extern char gdt_end[];
 extern char idtarg[];
+#endif
 
-/* These symbols are defined in secondary.S. */
-extern char _secondary_gdt_addr[];
-extern char _secondary_start[];
-extern char _secondary_start_end[];
+/* Offset to GDT's segment descriptors: */
+#define GDT_CODE_SEG            0x08
+#define GDT_DATA_SEG            0x10
+#define GDT_CODE_SEG64          0x18
+/*
+ * This define is placed here to make sure future romstage programmers
+ * know about it.
+ * It is used only in SMM for STM setup code.
+ */
+#define GDT_TASK_STATE_SEG      0x20
+
+#define GDT_CODE16_SEG          0x28
+#define GDT_DATA16_SEG          0x30
+#define GDT_DATA_ACPI_SEG       0x38
 
 #endif /* CPU_X86_GDT */