Add RTS option to disable writing of tix files

Summary

If we compile an executable Example with the -fhpc option, then a file Example.tix is written to disk after execution. We can change the name of the generated tix file with an environment variable HPCTIXFILE, but we cannot completely suppress the generation of the tix file. But we don't always need to generate this tix file, since the functions in Trace.Hpc.Reflect allow us to inspect the tix datastructures at runtime instead of looking at the generated file after execution. Generating the .tix files has downsides, since all tooling has to repeatedly delete the generated tix files, otherwise the executable crashes at startup time(Cp. https://github.com/ghc-proposals/ghc-proposals/pull/612).

So I suggest to add an RTS flag --write-tix-file=<yes|no> (which defaults to the current behaviour =yes). I have already implemented the flag, and will open a MR.