Fixing the reference declarations for System.MathF.

[tannergooding]

@janvorli. The mscorlib reference declarations should have been updated alongside: #7721

It also looks like the reference declarations should have used throw null; instead of return default(float);

Finally, it looks like I should have removed the System.Security.SecuritySafeCritical attribute from System.MathF.Abs since it is no longer an FCALL.

This fixes all three of those issues.

[tannergooding]

I went through all the code added in the original PR and ensured that there was no additional fixes required after System.MathF.Abs was modified to call the System.Math.Abs implementation.

[janvorli]

LGTM