include/arch: Steal native_posix bit manipulation routines

I was half way through typing up my own one of these when I realized
there was one already in the tree.  Move it to a shared header.

(FWIW: I really doubt that most architectures actually benefit from
their own versions of these tools -- GCC's optimizer is really good,
and custom assembly defeats optimization and factorizations of the
expressions in context.)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>