x86/mm: Move arch_memory_failure() and arch_is_platform_page() definitions from <asm/processor.h> to <asm/pgtable.h>

<linux/mm.h> relies on these definitions being included first,
which is true currently due to historic header spaghetti,
but in the future <asm/processor.h> will not guaranteed to be
included by the MM code.

Move these definitions over into a suitable MM header.

This is a preparatory patch for x86 header dependency simplifications
and reductions.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-kernel@vger.kernel.org