Add system-linux to allow tests run with llvm-lit to restrict themselves to linux

[jakehehrlich]

Summary

I need a test that only runs in a reasonable amount of time on systems that have sparse files. The broadest class of systems that support sparse files are linux systems. So restricting my test to linux systems should suffice. This change adds the system-linux feature to llvm-lit so that it can be required.

llvm/trunk/utils/lit/lit/llvm/config.py

def __init__(self, lit_config, config):
         lit_shell_env = os.environ.get('LIT_USE_INTERNAL_SHELL')
         if lit_shell_env:
             self.use_lit_shell = lit.util.pythonize_bool(lit_shell_env)
 
         if not self.use_lit_shell:
             features.add('shell')
 
         # Running on Darwin OS
-        if platform.system() in ['Darwin']:
+        if platform.system() == 'Darwin':
             # FIXME: lld uses the first, other projects use the second.
             # We should standardize on the former.
             features.add('system-linker-mach-o')
             features.add('system-darwin')
-        elif platform.system() in ['Windows']:
+        elif platform.system() == 'Windows':
             # For tests that require Windows to run.
             features.add('system-windows')
+        elif platform.system() == "Linux":
+            features.add('system-linux')
 
         # Native compilation: host arch == default triple arch
         # Both of these values should probably be in every site config (e.g. as
         # part of the standard header.  But currently they aren't)
         host_triple = getattr(config, 'host_triple', None)
         target_triple = getattr(config, 'target_triple', None)
         if host_triple and host_triple == target_triple:
             features.add('native')