Update printf %b/%B C2x support

WG14 recently accepted two additions to the printf/scanf %b/%B
support: there are now PRIb* and SCNb* macros in <inttypes.h>, and
printf %B is now an optional feature defined in normative text,
instead of recommended practice, with corresponding PRIB* macros that
can also be used to test whether that optional feature is supported.
See N3072 items 14 and 15 for details (those changes were accepted,
some other changes in that paper weren't).

Add the corresponding PRI* macros to glibc and update one place in the
manual referring to %B as recommended.  (SCNb* should naturally be
added at the same time as the corresponding scanf %b support.)

Tested for x86_64 and x86.