Subzero: Make python clean up after itself by removing its /tmp subdir.

pydir/build-pnacl-ir.py

 #!/usr/bin/env python2
 
 import argparse
+import errno
 import os
+import shutil
 import tempfile
 from utils import shellcmd
 from utils import FindBaseNaCl
@@ -21,6 +23,7 @@ if __name__ == '__main__':
         nacl_root + '/toolchain/linux_x86/pnacl_newlib/bin' + os.pathsep +
         os.pathsep + os.environ['PATH'])
 
+    try:
         tempdir = tempfile.mkdtemp()
 
         for cname in args.cfile:
@@ -36,3 +39,9 @@ if __name__ == '__main__':
                       ' -verify-pnaclabi-module -verify-pnaclabi-functions') +
                      ' -pnaclabi-allow-debug-metadata'
                      ' {0} -S -o {1}'.format(llname, pnaclname))
+    finally:
+        try:
+            shutil.rmtree(tempdir)
+        except OSError as exc:
+            if exc.errno != errno.ENOENT: # ENOENT - no such file or directory
+                raise # re-raise exception