x86/asm/entry/64: Clean up entry_64.S

Make the 64-bit syscall entry code a bit more readable:

 - use consistent assembly coding style similar to the other entry_*.S files

 - remove old comments that are not true anymore

 - eliminate whitespace noise

 - use consistent vertical spacing

 - fix various comments

 - reorganize entry point generation tables to be more readable

No code changed:

  # arch/x86/entry/entry_64.o:

   text    data     bss     dec     hex filename
  12282       0       0   12282    2ffa entry_64.o.before
  12282       0       0   12282    2ffa entry_64.o.after

md5:
   cbab1f2d727a2a8a87618eeb79f391b7  entry_64.o.before.asm
   cbab1f2d727a2a8a87618eeb79f391b7  entry_64.o.after.asm

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>