Make concurrently accessed globals atomic

Eliminate data races on gHaveTLS, gShutdown, and
gDisableBackgroundScheduling by declaring them as atomic. Use
explicit load() and store() operations to access them.

Note that gHaveTLS requires memory ordering guarantees, and thus,
even with a cooperative compiler, the old code would never have been
compiled to correct ARM object code.

Document a couple of weirdnesses remaining in the code.

Bug: 36697681
Test: Boot AOSP, TreeHugger
Change-Id: If44a10e31c1e091f06179154e07ced3da0a37b68

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