* Implement intrinsics for the Subzero backend required for
ANGLE / SwiftShader for GLES 2 and 3.
* Note that most intrinsics are implemented as "emulated". I've added
rr::emulated namespace in EmulatedReactor.hpp/cpp that contains the set
of Reactor functions that are presently being emulated. These are
invoked from SubzeroReactor until we decide to implement proper
intrinsics for these in Subzero.

Bug: b/130459196
Change-Id: I01171cfa7cc45b078c3b98be6b61328eee4f35e5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38874
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>