tinystdio: Mark ungetc using +1 instead of |0x100

Prepare for wide char support.  This allows use of nearly the full
range of __ungetc_t values, which means all of UCS2 (as 0xffff is
invalid). Still not all of a 4-byte wchar_t type, but it's better?

Signed-off-by: Keith Packard <keithp@keithp.com>

newlib/libc/tinystdio/fgetc.c

@@ -43,7 +43,7 @@ fgetc(FILE *stream)
 		return EOF;
 
 	if ((unget = __atomic_exchange_ungetc(&stream->unget, 0)) != 0)
-		return (unsigned char) unget;
+                return (unsigned char) (unget - 1);
 
 	rv = stream->get(stream);
 	if (rv < 0) {

newlib/libc/tinystdio/stdio_private.h

@@ -44,11 +44,6 @@
 #define SCANF_STD 1
 #define SCANF_FLT 2
 
-/* This is OR'd into the char stored in unget to make it non-zero to
- * flag the unget buffer as being full
- */
-#define UNGETC_MARK	0x0100
-
 struct __file_str {
 	struct __file file;	/* main file struct */
         char	*pos;		/* current buffer position */

newlib/libc/tinystdio/ungetc.c

@@ -44,7 +44,7 @@ ungetc(int c, FILE *stream)
 	if ((stream->flags & __SRD) == 0 || c == EOF)
 		return EOF;
 
-	if (!__atomic_compare_exchange_ungetc(&stream->unget, 0, c | UNGETC_MARK))
+	if (!__atomic_compare_exchange_ungetc(&stream->unget, 0, (__ungetc_t) c + 1 ))
 		return EOF;
 
         stream->flags &= ~__SEOF;