Subzero. Adds x86-64 to the list of supported Subzero targets.

Related changes:
NaCl change: https://codereview.chromium.org/1201483005
LLVM change: https://codereview.chromium.org/1193843016

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4077
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1199043006.
5 files changed