Change SystemZ large tests to use the existing long_tests property

[dblaikie]

Summary

(this is already used in Clang for a couple of tests)

[uweigand]

Should probably update the comment in test/CodeGen/SystemZ/Large/lit.local.cfg as well.
Otherwise LGTM.

test/CodeGen/SystemZ/Large/lit.local.cfg

 config.suffixes = ['.py']
 
 # These tests take on the order of seconds to run, so skip them unless
 # running natively.
-if config.root.host_arch not in ['SystemZ']:
+if 'long_tests' not in config.available_features:
     config.unsupported = True
 
 if not 'SystemZ' in config.root.targets:
     config.unsupported = True

test/lit.cfg

 if (config.llvm_use_sanitizer == "Memory" or
         config.llvm_use_sanitizer == "MemoryWithOrigins"):
     config.available_features.add("msan")
 if config.llvm_use_sanitizer == "Undefined":
     config.available_features.add("ubsan")
 else:
     config.available_features.add("not_ubsan")
 
+# Check if we should run long running tests.
+if lit_config.params.get("run_long_tests", None) == "true":
+    config.available_features.add("long_tests")
+
 # Direct object generation
 # Suppress x86_64-mingw32 while investigating since r219108.
 if not 'hexagon' in config.target_triple and not re.match(r'^x86_64.*-(mingw32|windows-gnu|win32)', config.target_triple):
     config.available_features.add("object-emission")
 
 if config.have_zlib == "1":
     config.available_features.add("zlib")
 else: