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

src/untrusted/irt/nacl.scons

Index: src/untrusted/irt/nacl.scons
diff --git a/src/untrusted/irt/nacl.scons b/src/untrusted/irt/nacl.scons
index df60f369d98e9270bff9abc222110c10103981e2..0fab43e459d3d874dd4d4ccd2ad17e570ec7a174 100644
--- a/src/untrusted/irt/nacl.scons
+++ b/src/untrusted/irt/nacl.scons
@@ -92,8 +92,17 @@ irt_libs = ['srpc',
             'm',
             ]
 
-irt_core_library = blob_env.ComponentProgram(
-    'irt_core', irt_support_objs + irt_nonbrowser, EXTRA_LIBS=irt_libs)
+irt_core_raw = blob_env.ComponentProgram(
+    'irt_core_raw', irt_support_objs + irt_nonbrowser, EXTRA_LIBS=irt_libs)
+
+irt_core_nexe = blob_env.ApplyTLSEdit(
+    'irt_core.nexe',
+    irt_core_raw)
+
+irt_core_library = blob_env.Install('${STAGING_DIR}', irt_core_nexe)
+blob_env.Alias('irt_core', irt_core_library)
+blob_env.ComponentProgramAlias(irt_core_library)
+
 env.SDKInstallBin('irt_core.nexe', irt_core_library)
 
 # TODO(mcgrathr): these should be installed, but scons is a mystery