These macros are often used by drivers and there exists already a lot of
duplication as ICPU() macro across the drivers.

Provide a generic x86 macro for users.

Note, as Ingo Molnar pointed out this has a hidden issue when a driver
needs to preserve const qualifier. Though, it would be addressed
separately at some point.
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Pavel Tatashin <pasha.tatashin@oracle.com>
Link: https://lkml.kernel.org/r/20180629193113.84425-2-andriy.shevchenko@linux.intel.com