Fix if...fi generation in the generated APE shell script

A shell will fail with a syntax error on an empty `if` or `else` body.
That is, neither of these is allowed:

    # Empty `if`
    if [ ... ]; then
    fi

    # Empty `else`
    if [ ... ]; then
    ...
    else
    fi

There were two places where `apelink` could generate problematic `if`'s:

1. The XNU shell generation for aarch64 binaries when no loaders (either
   binary or source) are provided. They can't assimilate, so the resulting
   `else` body becomes empty.
   There is actually a code path guarded by the `gotsome` variable that
   inserts an extra `true` in this case, but the variable was never
   initialized, so in practice this code path didn't activate in my
   tests. This is fixed by initializing the variable.
2. The loader extraction code when no loaders are provided and XNU
   support is requested. This is fixed by adding a simliar code path
   that prevents an empty body from being generated.

tool/build/apelink.c

@@ -2036,7 +2036,7 @@ int main(int argc, char *argv[]) {
     //      let our shell script compile the ape loader on first run.
     //
     if (support_vector & _HOSTXNU) {
-      bool gotsome;
+      bool gotsome = false;
       p = stpcpy(p, "else\n");  // if [ -d /Applications ]; then
 
       // output native mach-o morph
@@ -2136,6 +2136,7 @@ int main(int argc, char *argv[]) {
     }
 
     // extract the ape loader for open platforms
+    bool gotsome = false;
     if (inputs.n && (support_vector & _HOSTXNU)) {
       p = stpcpy(p, "if [ ! -d /Applications ]; then\n");
     }
@@ -2158,9 +2159,13 @@ int main(int argc, char *argv[]) {
                       "mv -f \"$t.$$\" \"$t\" ||exit\n");
         p = stpcpy(p, "exec \"$t\" \"$o\" \"$@\"\n"
                       "fi\n");
+        gotsome = true;
       }
     }
     if (inputs.n && (support_vector & _HOSTXNU)) {
+      if (!gotsome) {
+        p = stpcpy(p, "true\n");
+      }
       p = stpcpy(p, "fi\n");
     }