Suppress printing of placeholder external functions.

With the change to the parser to preserve indices, it now inserts
placeholders to pad out index spaces as needed. Placeholder functions
use reserved signature indices, so skip them when writing them out,
to avoid writing them out as "sig4294967295".

cranelift/filetests/parser/call.cton

@@ -33,6 +33,7 @@ function %signatures() {
 ; check:      sig10 = () native
 ; check:      sig11 = (i32, f64) -> i32, b1 spiderwasm
 ; check:      sig12 = (i32) -> b1 native
+; not:        fn0
 ; check:      fn5 = sig11 %foo
 ; check:      fn8 = sig12 %bar
 ; check:  }