Correct a small bug which throws a compile-time assertion for ARMv8
builds since this change:
  https://codereview.chromium.org/2223433002

This does not add full support for ARMv8 instructions; it simply sets
the CAN_USE_ARMV7_INSTRUCTIONS define on ARMv8 builds.

BUG=

Review-Url: https://codereview.chromium.org/2293253004
Cr-Commit-Position: refs/heads/master@{#39087}