[mlir][llvm] Add extra attributes to the atomic ops.

The revision adds a number of extra arguments to the
atomic read modify write and compare and exchange
operations. The extra arguments include the volatile,
weak, syncscope, and alignment attributes.

The implementation also adapts the fence operation to use
a assembly format and generalizes the helper used
to obtain the syncscope name.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D143554