[NaCl SDK] Enable sel_ldr_x86_64 in mac SDK.

BUG=276662
R=binji@chromium.org

Review URL: https://codereview.chromium.org/36053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231044 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231044
faq link for security contest, restified version

BUG=None
TEST=None
R=eliben@chromium.org

Review URL: https://codereview.chromium.org/40323005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231045 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231045