printf core: replace if(0) blocks around switch cases with explicit gotos

this is purely a readability change, not a functional one. all of the
integer format cases use a common tail for handling precision logic
after the string representation of the number has been generated. the
code as I originally wrote it was overly clever in the aim of making a
point that the flow could be done without goto, and jumped over
intervening cases by wrapping them in if (0) { }, with the case labels
for each inside the conditional block scope.

this has been a perpetual source of complaints about the readability
and comprehensibility of the file, so I am now changing it to
explicitly jump to the tail logic with goto statements.

src/stdio/vfprintf.c

@@ -557,11 +557,11 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 		case 'x': case 'X':
 			a = fmt_x(arg.i, z, t&32);
 			if (arg.i && (fl & ALT_FORM)) prefix+=(t>>4), pl=2;
-			if (0) {
+			goto ifmt_tail;
 		case 'o':
 			a = fmt_o(arg.i, z);
 			if ((fl&ALT_FORM) && p<z-a+1) p=z-a+1;
-			} if (0) {
+			goto ifmt_tail;
 		case 'd': case 'i':
 			pl=1;
 			if (arg.i>INTMAX_MAX) {
@@ -573,7 +573,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 			} else pl=0;
 		case 'u':
 			a = fmt_u(arg.i, z);
-			}
+		ifmt_tail:
 			if (xp && p<0) goto overflow;
 			if (xp) fl &= ~ZERO_PAD;
 			if (!arg.i && !p) {