[lit] Add pthread to the compilation of the tests on Linux

[stella.stamenova]

Summary

Right now only some platforms add pthread to the compilation, however, at least one of the tests requires the pthread library on Linux as well. Since the library is available, this change adds it by default on Linux.

[zturner]

Lgtm

lit/helper/toolchain.py

if platform.system() in ['Darwin']:
             out = subprocess.check_output(['xcrun', '--show-sdk-path']).strip()
             res = 0
         except OSError:
             res = -1
         if res == 0 and out:
             sdk_path = lit.util.to_string(out)
             lit_config.note('using SDKROOT: %r' % sdk_path)
             flags = ['-isysroot', sdk_path]
-    elif platform.system() in ['OpenBSD']:
+    elif platform.system() in ['OpenBSD', 'Linux']:
         flags = ['-pthread']
 
 
     additional_tool_dirs=[]
     if config.lldb_lit_tools_dir:
         additional_tool_dirs.append(config.lldb_lit_tools_dir)
 
     llvm_config.use_clang(additional_flags=flags,