Change i386 switch_to to be unwinder friendly

The dwarf2 unwinder would always get stuck in context_switch() in
the inline assembly switch_to() macro which is putting
data on the stack without properly dwarf2 annotating it.
Move the flags save/restore to C code to avoid that.

Signed-off-by: Andi Kleen <ak@suse.de>

Index: linux/include/asm-x86/system_32.h
===================================================================
--- linux.orig/include/asm-x86/system_32.h
+++ linux/include/asm-x86/system_32.h
@@ -18,20 +18,20 @@ extern struct task_struct * FASTCALL(__s
  */
 #define switch_to(prev,next,last) do {					\
 	unsigned long esi,edi;						\
-	asm volatile("pushfl\n\t"		/* Save flags */	\
-		     "pushl %%ebp\n\t"					\
+	unsigned long flags = native_save_fl();				\
+	asm volatile("pushl %%ebp\n\t"					\
 		     "movl %%esp,%0\n\t"	/* save ESP */		\
 		     "movl %5,%%esp\n\t"	/* restore ESP */	\
 		     "movl $1f,%1\n\t"		/* save EIP */		\
 		     "pushl %6\n\t"		/* restore EIP */	\
 		     "jmp __switch_to\n"				\
 		     "1:\t"						\
-		     "popl %%ebp\n\t"					\
-		     "popfl"						\
+		     "popl %%ebp\n"					\
 		     :"=m" (prev->thread.esp),"=m" (prev->thread.eip),	\
 		      "=a" (last),"=S" (esi),"=D" (edi)			\
 		     :"m" (next->thread.esp),"m" (next->thread.eip),	\
 		      "2" (prev), "d" (next));				\
+	native_restore_fl(flags);					\
 } while (0)
 
 #define _set_base(addr,base) do { unsigned long __pr; \