ART: Fix mac build

is_main_thread is unused in the case of a mac.

Bug: 15435566

(cherry picked from commit 7b1bf42dbc765524d54d45a70cb7eb1cccf0c617)

Change-Id: I9848d0156a4903ce73d20b5ce282b61ceb3039ff
diff --git a/runtime/ b/runtime/
index 4ce8c96..b2fbf74 100644
--- a/runtime/
+++ b/runtime/
@@ -505,9 +505,7 @@
   // TODO: move this into the Linux GetThreadStack implementation.
-#if defined(__APPLE__)
-  bool is_main_thread = false;
+#if !defined(__APPLE__)
   // If we're the main thread, check whether we were run with an unlimited stack. In that case,
   // glibc will have reported a 2GB stack for our 32-bit process, and our stack overflow detection
   // will be broken because we'll die long before we get close to 2GB.