These used a static for caching the computation, but didn't use a
static initializer. This meant there was a race between the check
for zero and the assignment.

Bug: b/153803432
Change-Id: Id5b91050ced0b8d4811eb32ffed24885816ade7a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43909Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>