[mlir][math][spirv] Add `math.roundeven` lowering to SPIR-V

This has two lowering path, one for each extended instructions set:
-  to OpenGL's `RoundEven`,
-  to OpenCL's `rint`.

Implement those two ops and add minimal tests.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D137171