Print newlines only for lines not to be included

- to preserve line number info for mapping setting to original source code
- still removes "# vim:" lines (as they are at the end of the file anyways)
- still adds a "$# vim:..." line at the end, to force non-modifiability

Inspired by http://perl6.party/post/Perl-6-Core-Hacking-Wheres-Da-Sauce-Boss
by Zoffix++

tools/build/gen-cat.nqp

@@ -30,6 +30,7 @@ sub MAIN(*@ARGS) {
                 nqp::die("Nested conditionals not supported") if $in_cond;
                 $in_cond := 1;
                 $in_omit := $x[0] && $x[1] eq $backend || !$x[0] && $x[1] ne $backend;
+                print("\n");
             } elsif $_ ~~ /^ '#?endif' / {
                 unless $in_cond {
                     nqp::sayfh($stderr,
@@ -38,8 +39,11 @@ sub MAIN(*@ARGS) {
                 }
                 $in_cond := 0;
                 $in_omit := 0;
-            } elsif  !$in_omit {
-                print($_) unless $_ ~~ /^ '# ' \w /;
+                print("\n");
+            } elsif $in_omit {
+                print("\n");
+            } else {
+                print($_) unless nqp::eqat($_,"# vim:",0);
             }
             $line++;
         }