lit: Hardcode whence seek value, os.SEEK_END isn't always available.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@86449 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/lit/TestRunner.py b/utils/lit/TestRunner.py
index 003bcc8..5b4538f 100644
--- a/utils/lit/TestRunner.py
+++ b/utils/lit/TestRunner.py
@@ -114,7 +114,7 @@
                         r[2] = open(r[0], r[1])
                     # Workaround a Win32 and/or subprocess bug when appending.
                     if r[1] == 'a':
-                        r[2].seek(0, os.SEEK_END)
+                        r[2].seek(0, 2)
                 result = r[2]
             final_redirects.append(result)