x86/entry/common: Provide idtentry_enter/exit()

Provide functions which handle the low level entry and exit similar to
enter/exit from user mode.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505134904.457578656@linutronix.de

arch/x86/include/asm/idtentry.h

@@ -7,6 +7,9 @@
 
 #ifndef __ASSEMBLY__
 
+void idtentry_enter(struct pt_regs *regs);
+void idtentry_exit(struct pt_regs *regs);
+
 /**
  * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
  *		      No error code pushed by hardware