After a hiatus while x32 was not available in nacl's llvm, x8664 is
being revived. Rejoice!

The Target is now back to where it was before: the crosstests pass, and
SPEC2k builds and verifies.

Makefile.standalone still has the crosstests for x8664 disabled while
we wait for all the plumbing that's needed for x32 support on nacl's
toolchain to be available.

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

Review URL: https://codereview.chromium.org/1537703002 .