Subzero. Native 64-bit int arithmetic on x86-64.

This CL modifies the x86 instruction selection template to allow native
64-bit GPR support. It also enables x86-64 crosstests.

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

Review URL: https://codereview.chromium.org/1273153002.