Enable flock syscall for NaCl processes

In order to make pwrite obey POSIX semantics on Linux, the NaCl
runtime need to be able to flock the file to temporarily remove the
O_APPEND flag.  The file lock is used to prevent two NaCl processes
from seeing the temporary lack of O_APPEND at regular write syscalls.

R=jln@chromium.org

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3441
TEST= None

Review URL: https://codereview.chromium.org/27272003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228942 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228942