ldbl-128ibm-compat: Add obstack printing functions

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>