Clean up use of HAVE_ANDROID_OS

HAVE_ANDROID_OS was defined as "1" for targets, but never defined as "0"
for non-targets. Changing them to #ifdef should be safe and matches all
the other uses of HAVE_ANDROID_OS throughout the system.

Change-Id: I077aee39b08ce82327a571d9aca0cae422a0ebb5
diff --git a/vm/native/dalvik_system_Zygote.c b/vm/native/dalvik_system_Zygote.c
index bcc2313..c568005 100644
--- a/vm/native/dalvik_system_Zygote.c
+++ b/vm/native/dalvik_system_Zygote.c
@@ -299,7 +299,7 @@
 #endif
     }
 
-#if HAVE_ANDROID_OS
+#ifdef HAVE_ANDROID_OS
     if ((debugFlags & DEBUG_ENABLE_DEBUGGER) != 0) {
         /* To let a non-privileged gdbserver attach to this
          * process, we must set its dumpable bit flag. However