[SPARC][IAS] Add aliases for %asr20-21 as defined in JPS1

This adds %set_softint and %clear_softint alias for %asr20 and %asr21
as defined in JPS1.

Reviewers: jrtc27, brad0, s-barannikov, rorth

Reviewed By: s-barannikov

Pull Request: https://github.com/llvm/llvm-project/pull/94247