Subzero: Run cross tests as a much more configurable python script.

crosstest/lit.cfg

+import os
+import re
+import lit.formats
+
+config.name = 'subzero_crosstests'
+config.test_format = lit.formats.ShTest()
+config.suffixes = ['.xtest']
+config.test_source_root = os.path.dirname(__file__)
+config.test_exec_root = config.test_source_root
+
+llvmbintools = [r"\bFileCheck\b"]
+llvmbinpath = os.path.abspath(os.environ.get('LLVM_BIN_PATH'))
+
+for tool in llvmbintools:
+  # The re.sub() line is adapted from one of LLVM's lit.cfg files.
+  # Extract the tool name from the pattern.  This relies on the tool
+  # name being surrounded by \b word match operators.  If the
+  # pattern starts with "| ", include it in the string to be
+  # substituted.
+  substitution = re.sub(r"^(\\)?((\| )?)\W+b([0-9A-Za-z-_]+)\\b\W*$",
+                        r"\2" + llvmbinpath + "/" + r"\4",
+                        tool)
+  config.substitutions.append((tool, substitution))