Adds tls_edit utility which patches irt_core.nexe's TLS usage.

tests/irt_private_pthread/nacl.scons

Index: tests/irt_private_pthread/nacl.scons
diff --git a/tests/irt_private_pthread/nacl.scons b/tests/irt_private_pthread/nacl.scons
index d89161b976852b1497eda1de9d7e3d2f2e577936..2aa8f3c16ecf568171fc037f5a3d2c84146dd786 100644
--- a/tests/irt_private_pthread/nacl.scons
+++ b/tests/irt_private_pthread/nacl.scons
@@ -17,9 +17,18 @@ if env.Bit('bitcode'):
 #    __pthread_initialize() and the intercepted malloc() uses TLS.
 is_broken = env.Bit('running_on_valgrind')
 
-irt_thread_test_nexe = env.ComponentProgram(
-    'irt_thread_test', ['irt_thread_test.c'],
+irt_thread_test_raw = env.ComponentProgram(
+    'irt_thread_test_raw', ['irt_thread_test.c'],
     EXTRA_LIBS=['irt_support_private'])
+
+tls_edit_exe = env.GetTLSEdit()
+
+irt_thread_test_nexe = env.Command(
+    'irt_thread_test.nexe',
+    [tls_edit_exe, irt_thread_test_raw],
+    '${SOURCES} ${TARGET}'
+    )
+
 node = env.CommandSelLdrTestNacl('irt_thread_test.out', irt_thread_test_nexe)
 env.AddNodeToTestSuite(node, ['small_tests'], 'run_irt_thread_test',
                        is_broken=is_broken)