Similarly to the functions from the *printf family, this patch adds
implementations for __obstack_*printf* functions that set the
'mode_flags' parameter to PRINTF_LDBL_USES_FLOAT128, before making calls
to __vfprintf_internal (indirectly through __obstack_vprintf_internal).
Tested for powerpc64le.
Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>
