Enable genFnCalleeRegArgs for Arm64 Varargs (dotnet/coreclr#18714)

Enable genFnCalleeRegArgs for Arm64 Varargs (dotnet/coreclr#18714)

* Enable genFnCalleeRegArgs for Arm64 Varargs

Before the method would early out and incorrectly expect the usage
of all incoming arguments to be their homed stack slots. It is
instead possible for incoming arguments to be homed to different
integer registers.

The change will mangle the float types for vararg cases in the same
way that is done during lvaInitUserArgs and fgMorphArgs.

* Apply format patch

* Account for softfp case

* Address feedback

* Apply format patch

* Use standard function header for mangleVarArgsType

* Remove confusing comment

Commit migrated from https://github.com/dotnet/coreclr/commit/b1bda1bbc9a5fe9954938b4b627660d3acbe7504