stdio: Legacy stdio only handles 80-bit long double

Skip all other forms for long double as the code doesn't handle them
correctly, not even 64-bit long double.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

newlib/libc/stdio/vfscanf.c

@@ -130,7 +130,7 @@ Supporting OS subroutines required:
    This could be changed in the future should the __ldtoa code be
    preferred over __dtoa.  */
 #define _NO_LONGDBL
-#if defined _WANT_IO_LONG_DOUBLE && (LDBL_MANT_DIG > DBL_MANT_DIG)
+#if defined _WANT_IO_LONG_DOUBLE && (LDBL_MANT_DIG == 64)
 #undef _NO_LONGDBL
 #endif