x86: make 'constant_test_bit()' take an unsigned bit number

Ingo noticed that using signed arithmetic seems to confuse the gcc
inliner, and make it potentially decide that it's all too complicated.

(Yeah, yeah, it's a constant. It's always positive. Still..)

Based-on: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>