x86/cpufeature: Add helper macro for mask check macros

Every time we add a word to our cpu features, we need to add
something like this in two places:

	(((bit)>>5)==16 && (1UL<<((bit)&31) & REQUIRED_MASK16))

The trick is getting the "16" in this case in both places.  I've
now screwed this up twice, so as pennance, I've come up with
this patch to keep me and other poor souls from doing the same.

I also commented the logic behind the bit manipulation showcased
above.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20160629200110.1BA8949E@viggo.jf.intel.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>