Subzero: The cross tests should use the actual Subzero runtime.

pydir/crosstest.py

 #!/usr/bin/env python2
 
 import argparse
 import os
 import re
 import subprocess
 import sys
 import tempfile
 
 from utils import shellcmd
@@ skipping 104 matching lines @@
         # approaches can produce different semantics on some undefined
         # bitcode behavior.  Specifically, LLVM produces different
         # results for overflowing fptoui instructions for i32 and i64
         # on x86-32.  As it turns out, Subzero lowering was based on
         # inspecting the object code produced by the direct llc
         # command, so we need to directly run llc on the bitcode, even
         # though it makes this script longer, to avoid spurious
         # failures.  This behavior can be inspected by switching
         # use_llc between True and False.
         use_llc = False
+        pure_c = os.path.splitext(args.driver)[1] == '.c'
         if not args.crosstest_bitcode:
             objs.append(arg)
         elif use_llc:
             shellcmd(['llc'
                       '-filetype=obj',
                       '-o=' + obj_llc,
                       bitcode])
             objs.append(obj_llc)
         else:
             objs.append(bitcode)
 
-    linker = 'clang' if os.path.splitext(args.driver)[1] == '.c' else 'clang++'
+    # Use 'clang szrt.c'  -or-  'clang++ szrt.cpp'
+    objs.append((
+            '{root}/toolchain_build/src/subzero/runtime/szrt.{ext}'
+            ).format(root=nacl_root, ext='c' if pure_c else 'cpp'))
+    linker = 'clang' if pure_c else 'clang++'
     shellcmd([linker, '-g', '-m32', args.driver] +
              objs +
              ['-lm', '-lpthread', '-o', os.path.join(args.dir, args.output)])