Add mips64 to the makefile.

Currently we have no optimizations for mips64, so it correctly
contains the various _none.cpps.

Corresponds to https://android-review.googlesource.com/#/c/96536/1

R=halcanary@google.com, djsollen@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/311163003
1 file changed