Adding some new functions to System.Math and System.MathF (dotnet/coreclr#20788)

Adding some new functions to System.Math and System.MathF (dotnet/coreclr#20788)

* Adding BitIncrement, BitDecrement, CopySign, MaxMagnitude, and MinMagnitude to Math and MathF

* Adding FusedMultiplyAdd, IlogB, Log2, and ScaleB to Math and MathF

* Adding some basic PAL tests for fma, ilogb, log2, and scalbn

* Fixing a couple typos and adding clarifying comments

* Fixing the MSVC _VVV FCALL declarations

Commit migrated from https://github.com/dotnet/coreclr/commit/28417584d8e98ae7eac22e92b952778f8ea94047