Make public property for setting illinker special options

[marek-safar]

It's useful to have documented way to tweak illink task additional options. Today there is _ExtraTrimmerArgs property which does that but it needs to be "exposed", documented and probably renamed.

@sbomer

[sbomer]

That option is good for playing with experimental/advanced options, but if we make it "public", the supported/document command-line switches will need to be stable. I would prefer to expose any stable command-line switches as separate task inputs, like the Csc task does.

[joeloff]

I think it's reasonable to expose and make _ExtraTrimmerArgs overridable. As the ILLink task makes options official, we'll likely expose those as individual properties like @sbomer suggets