fmt,cxxrtl: add support for NUMERIC justification.

Before this commit, the existing alignments were `LEFT` and `RIGHT`,
which added the `padding` character to the right and left just before
finishing formatting. However, if `padding == '0'` and the alignment is
to the right, then the padding character (digit zero) was added after
the sign, if one is present.

After this commit, the special case for `padding == '0'` is removed,
and the new justification `NUMERIC` adds the padding character like
the justification `RIGHT`, except after the sign, if one is present.
(Space, for the `SPACE_MINUS` sign mode, counts as the sign.)

kernel/fmt.h

@@ -69,6 +69,7 @@ struct FmtPart {
 	enum {
 		RIGHT	= 0,
 		LEFT	= 1,
+		NUMERIC	= 2,
 	} justify = RIGHT;
 	char padding = '\0';
 	size_t width = 0;