[mlir][spirv] Add path for math.round to spirv for OCL and GLSL

OpenCL's round function matches `math.round` so we can directly lower to
the op, this includes adding the op definition to the SPIRV OCL ops.
GLSL does not guarantee rounding direction so we include custom rounding
code to guarantee correct rounding direction.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D129236