[Bug libstdc++/51772] --enable-clocale=generic makes unsafe assumptions about ctype_base::mask

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51772

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
In config/locale/newlib/ctype_members.cc:

      default:
    // Different from the generic version, xdigit and print in
    // newlib are defined as bitwise-OR result of bitmasks:
    //   xdigit = _X | _N;
    //   print  = _P | _U | _L | _N | _B;
    // in which _X and _B don't correspond to any ctype mask.
    // In order to get the wmask correctly converted when __m is
    // equal to _X or _B, the two cases are specifically handled
    // here.
    if (__m & xdigit)
      __ret = wctype("xdigit");
    else if (__m & print)
      __ret = wctype("print");
    else
      __ret = __wmask_type();
      }

This doesn't only apply to the masks defined in config/os/newlib/ctype_base.h,
but also netbsd and openbsd and bionic.

Something similar is also needed for qnx, as config/os/qnx/qnx6.1/ctype_base.h
uses several bitmasks that do not correspond to any ctype, e.g. _XA, _SP, _XS,
_XB