Using 'volatile' does not make anything thread safe.

Shuts up a bunch of TSAN errors.

Bug: b/133127573
Change-Id: I90b708ac53824de91c316f61f137ecca34087047
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31830
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>