x86/entry: Distangle idtentry

idtentry is a completely unreadable maze. Split it into distinct idtentry
variants which only contain the minimal code:

  - idtentry for regular exceptions
  - idtentry_mce_debug for #MCE and #DB
  - idtentry_df for #DF

The generated binary code is equivalent.
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/20200505134903.949227617@linutronix.de