There are a lot more printf variants than there are scanf variants,
and the code for setting up and tearing down their custom FILE
variants around the call to __vf(w)printf is more complicated and
variable.  Therefore, I have added _internal versions of all the
v*printf variants, rather than introducing helper routines so that
they can all directly call __vf(w)printf_internal, as was done with
scanf.

As with the scanf changes, in this patch the _internal functions still
look at the environmental mode bits and all callers pass 0 for the
flags parameter.

Several of the affected public functions had _IO_ name aliases that
were not exported (but, in one case, appeared in libio.h anyway);
I was originally planning to leave them as aliases to avoid having
to touch internal callers, but it turns out ldbl_*_alias only work
for exported symbols, so they've all been removed instead.  It also
turns out there were hardly any internal callers.  _IO_vsprintf and
_IO_vfprintf *are* exported, so those two stick around.

Summary for the changes to each of the affected symbols:

  _IO_vfprintf, _IO_vsprintf:
    All internal calls removed, thus the internal declarations, as well
    as uses of libc_hidden_proto and libc_hidden_def, were also removed.
    The external symbol is now exposed via uses of ldbl_strong_alias
    to __vfprintf_internal and __vsprintf_internal, respectively.

  _IO_vasprintf, _IO_vdprintf, _IO_vsnprintf,
  _IO_vfwprintf, _IO_vswprintf,
  _IO_obstack_vprintf, _IO_obstack_printf:
    All internal calls removed, thus declaration in internal headers
    were also removed.  They were never exported, so there are no
    aliases tying them to the internal functions.  I.e.: entirely gone.

  __vsnprintf:
    Internal calls were always preceded by macros such as
      #define __vsnprintf _IO_vsnprintf, and
      #define __vsnprintf vsnprintf
    The macros were removed and their uses replaced with calls to the
    new internal function __vsnprintf_internal.  Since there were no
    internal calls, the internal declaration was also removed.  The
    external symbol is preserved with ldbl_weak_alias to ___vsnprintf.

  __vfwprintf:
    All internal calls converted into calls to __vfwprintf_internal,
    thus the internal declaration was removed.  The function is now a
    wrapper that calls __vfwprintf_internal.  The external symbol is
    preserved.

  __vswprintf:
    Similarly, but no external symbol.

  __vasprintf, __vdprintf, __vfprintf, __vsprintf:
    New internal wrappers.  Not exported.

  vasprintf, vdprintf, vfprintf, vsprintf, vsnprintf,
  vfwprintf, vswprintf,
  obstack_vprintf, obstack_printf:
    These functions used to be aliases to the respective _IO_* function,
    they are now aliases to their respective __* functions.

Tested for powerpc and powerpc64le.