Documented '-m' flags for machine specific instruction extensions.
Documented -m* flags (-msse3, etc.).
Some caveats: I added a note, that SSE2 will now be used as a default for both X86 and X86_64. This was changed in this commit and is not part of any release yet. So I wrote "since GHC 9.0.1", because I think that is the next release?
Furthermore, the existing flags had a note like "added in GHC 7.0.1", which I omitted. Figuring this out is a lot of work and I'm not sure how important that is? Especially since all the flags I have documented basically do nothing...
Tracking: #18641