[mlir][emitc] Add div, mul and rem operators

This adds operations for binary multiplicative arithmetic operators to
EmitC. The input and output arguments for the remainder operator are
restricted to index (emitted as size_t), integers and the EmitC opaque
types (as the operator can be overloaded for a custom type). The
multiplication and division operator further support floating point
numbers.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D154846