GDB serial and backtrace support.

* grub-core/kern/i386/realmode.S (real_to_prot): Reload IDT.
	(prot_to_real): Likewise.
	* grub-core/kern/i386/int.S (grub_bios_interrupt): Remove IDT reload.
	* grub-core/Makefile.core.def (backtrace): New module.
	(gdb): Likewise.
	* grub-core/gdb/cstub.c: New file.
	* grub-core/gdb/gdb.c: Likewise.
	* grub-core/gdb/i386/idt.c: Likewise.
	* grub-core/gdb/i386/machdep.S: Likewise.
	* grub-core/gdb/i386/signal.c: Likewise.
	* grub-core/lib/i386/backtrace.c: Likewise.
	* include/grub/backtrace.h: Likewise.
	* include/grub/gdb.h: Likewise.
	* include/grub/i386/gdb.h: Likewise.

	Also-By: Vladimir Serbinenko <phcoder@gmail.com>

grub-core/kern/i386/int.S

@@ -19,9 +19,6 @@
 FUNCTION(grub_bios_interrupt)
 	pushf
 	cli
-#ifndef GRUB_MACHINE_PCBIOS
-	sidt protidt
-#endif
 	popf
 	pushl    %ebp
 	pushl    %ecx
@@ -51,9 +48,6 @@ FUNCTION(grub_bios_interrupt)
 	.code16
 	pushf
 	cli
-#ifndef GRUB_MACHINE_PCBIOS
-	lidt realidt
-#endif
 	
 	mov	%ds, %ax
 	push	%ax
@@ -126,15 +120,4 @@ intno:
 	popl    %eax
 	popl    %ecx
 	popl    %ebp
-#ifndef GRUB_MACHINE_PCBIOS
-	lidt protidt
-#endif
 	ret
-#ifndef GRUB_MACHINE_PCBIOS
-realidt:
-	.word 0x100
-	.long 0
-protidt:
-	.word 0
-	.long 0
-#endif

grub-core/kern/i386/realmode.S

@@ -118,6 +118,12 @@ gdt:
 gdtdesc:
 	.word	0x27			/* limit */
 	.long	gdt			/* addr */
+realidt:
+	.word 0
+	.long 0
+protidt:
+	.word 0
+	.long 0
 
 /*
  *  These next two routines, "real_to_prot" and "prot_to_real" are structured
@@ -169,6 +175,9 @@ protcseg:
 	/* zero %eax */
 	xorl	%eax, %eax
 
+	sidt realidt
+	lidt protidt
+
 	/* return on the old (or initialized) stack! */
 	ret
 /*
@@ -195,6 +204,9 @@ prot_to_real:
 	/* just in case, set GDT */
 	lgdt	gdtdesc
 
+	sidt    protidt
+	lidt    realidt
+
 	/* save the protected mode stack */
 	movl	%esp, %eax
 	movl	%eax, protstack