[OpenMP][mlir] Lowering for omp.atomic.update

This patch adds lowering from omp.atomic.update to LLVM IR. Whenever a
special LLVM IR instruction is available for the operation, `atomicrmw`
instruction is emitted, otherwise a compare-exchange loop based update
is emitted.

Depends on D119522

Reviewed By: ftynse, peixin

Differential Revision: https://reviews.llvm.org/D119657