Make Linker.isUsed() static, and clean up the fallout.

Make Linker.isUsed() static, and clean up the fallout.

Make Linker.isUsed() static, so that client code no longer needs to
instantiate a Linker simply to find out that one is not used.
Clean up call sites to avoid nugatory instantiations.

Fix a race that occurred in testing code (prevalent on arm64 but
unseen on arm32) because of the previous non-static Linker.isUsed(),
add Linker testing convenience setup function, and clean up client
code that must configure a Linker for testing if required.

Enable testing for arm64, and check that it now works cleanly.

Tidy up a few method names and string formatting. Add a previously
missing synchronized(singletonLock).

BUG=385553

Review URL: https://codereview.chromium.org/1314823003

Cr-Commit-Position: refs/heads/master@{#346700}