[mlir][math] Initial support for fastmath flag attributes for Math dialect.

Added arith::FastMathAttr and ArithFastMathInterface support for Math dialect
floating point operations.

This change-set creates ArithCommon conversion utils that currently
provide classes and methods to aid with arith::FastMathAttr conversion
into LLVM::FastmathFlags. These utils are used in ArithToLLVM and
MathToLLVM convertors, but may eventually be used by other converters
that need to convert fast math attributes.

Since Math dialect operations use arith::FastMathAttr, MathOps.td now
has to include enum and attributes definitions from Arith dialect.
To minimize the amount of TD code included from Arith dialect,
I moved FastMathAttr definition into ArithBase.td.

Differential Revision: https://reviews.llvm.org/D136312