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Currently the Microarchitecture table in cpu_identity only supports Intel
Microarchitectures.  Add an entry for the AMD Excavator microarchitecture.

BUG=b:69376360
TEST=unit_tests --gtest_filter="CpuIdentityTest.*"
R=chongjiang@chromium.org,sque@chromium.org

Change-Id: I255596e330e0353ab4bec2d51ea25d2420cbcb30
Reviewed-on: https://chromium-review.googlesource.com/1033457Reviewed-by: Simon Que <sque@chromium.org>
Commit-Queue: Daniel Kurtz <djkurtz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555469}