Add new rounding modes to System.Math, System.MathF (dotnet/coreclr#20815)

Add new rounding modes to System.Math, System.MathF (dotnet/coreclr#20815)

* add new rounding modes to MidpointRounding.cs

new modes added to enum
implemented ToZero for double in Math.cs

* ToZero implementation

* implement double and float rounding modes

* updating rounding implementation

now round inline with DecCalc internal round implementation

* small bug fix

also replace var to make things obvious

* update implementation - floor/ceil

code review feedback

* review feedback

add comments, update MathF with floor/ceil

* code review feedback

 - fix comments
 - replace ifelse with switch
 - remove RoundingMode enum from DecCalc

* exclude outdated corefx test

Commit migrated from https://github.com/dotnet/coreclr/commit/3397472200fc7b2b5db3dfd27a652a12831c37ee