x86_64: add a helper function that displaces an address by a given amount.

The new function will be used in an follow up CL.

Add a gtest that constructs addresses with some initial displacement and
checks that they are equal to addresses constructed in the same way only
without displacement, and then displaced.

Bug: 65872996
Test: m test-art-host-gtest  # changes covered by the new test
Change-Id: Id282707077b513bf5911afad1cdd481a1611b55c
2 files changed