Use SCANF_LDBL_IS_DBL instead of __ldbl_is_dbl.

Change the callers of __vfscanf_internal and __vfwscanf_internal that
want to treat 'long double' as another name for 'double' (all of which
happen to be in sysdeps/ieee754/ldbl-opt/nldbl-compat.c) to communicate
this via the new flags argument, instead of the per-thread variable
__no_long_double and its __ldbl_is_dbl wrapper macro.

Tested for powerpc and powerpc64le.