Linus reported the following new warning on x86 allmodconfig with GCC 5.1:

  > ./arch/x86/include/asm/spinlock.h: In function ‘arch_spin_lock’:
  > ./arch/x86/include/asm/spinlock.h:119:3: warning: implicit declaration
  > of function ‘__ticket_lock_spinning’ [-Wimplicit-function-declaration]
  >    __ticket_lock_spinning(lock, inc.tail);
  >    ^

This warning triggers because of these hacks in misc.h:

  /*
   * we have to be careful, because no indirections are allowed here, and
   * paravirt_ops is a kind of one. As it will only run in baremetal anyway,
   * we just keep it from happening
   */
  #undef CONFIG_PARAVIRT
  #undef CONFIG_KASAN

But these hacks were not updated when CONFIG_PARAVIRT_SPINLOCKS was added,
and eventually (with the introduction of queued paravirt spinlocks in
recent kernels) this created an invalid Kconfig combination and broke
the build.

So add a CONFIG_PARAVIRT_SPINLOCKS #undef line as well.

Also remove the _ASM_X86_DESC_H quirk: that undocumented quirk
was originally added ages ago, in:

  099e1377

 ("x86: use ELF format in compressed images.")

and I went back to that kernel (and fixed up the main Makefile
which didn't build anymore) and checked what failure it
avoided: it avoided an include file dependencies related
build failure related to our old x86-platforms code.

That old code is long gone, the header dependencies got cleaned
up, and the build does not fail anymore with the totality of
asm/desc.h included - so remove the quirk.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>