lib9: format %#04x, 0 as 0x0000 not 000000.

This matches Go's fmt.Printf instead of ANSI C's dumb rules.
It makes the -S output from C liblink match Go's liblink.

LGTM=minux
R=minux
CC=golang-codereviews
https://golang.org/cl/112600043

src/lib9/fmt/dofmt.c

@@ -491,12 +491,6 @@ __ifmt(Fmt *f)
 			if(fl & FmtApost)
 				__needsep(&ndig, &grouping);
 		}
-
-		/*
-		 * Zero values don't get 0x.
-		 */
-		if(f->r == 'x' || f->r == 'X')
-			fl &= ~(ulong)FmtSharp;
 	}
 	for(w = f->prec; n < w && p > buf+3; n++){
 		if((fl & FmtApost) && __needsep(&ndig, &grouping)){