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* exclude outdated corefx test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