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