Add [[noreturn]] attribute to SST::Output::fatal()

[leekillough]

Describe the solution you'd like Add [[noreturn]] attribute to SST::Output::fatal()

[[noreturn]] allows greater optimization and avoids warnings about missing return values in functions which call SST::Output::fatal() but don't return values.

Describe alternatives you've considered Add dummy return after calls to SST::Output::fatal() to avoid warnings