for people who have a gcc that was configured with some non-default --with-arch but who want to build a generic binary, an option like -march=generic would be useful. It seems that the right spelling is -march=x86-64, but I couldn't find that documented in invoke.texi.
Yes, I only learned about it from reading the sources.
I just learned from it by googling! :(
If someone cared to explain it, I will add it to the FAQ (I can't send patches ATM, sorry)