* Most use Subzero intrinsics, except for Min/MaxAtomic, which are
emulated.
* Added unit tests for each implemented function, but am not really
testing that they behave as atomic functions. Only that they perform the
expected operation.

Bug: b/145754674
Change-Id: Ie3ec6e473ee8b448b28bf440da094ac03ac0005b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39829Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>