ldbl-128ibm-compat: Add wide character scanning functions

Similarly to what was done for regular character scanning functions,
this patch uses the new mode mask, SCANF_LDBL_USES_FLOAT128, in the
'mode' argument of the wide characters scanning function,
__vfwscanf_internal (which is also extended to support scanning
floating-point values with IEEE binary128, by redirecting calls to
__wcstold_internal to __wcstof128_internal).

Tested for powerpc64le.

Reviewed-By: Paul E. Murphy <murphyp@linux.ibm.com>