-C target-cpu should offer a 'native' option

[huonw]

Currently -C target-cpu just takes a raw list of LLVM's known CPU codes (see rustc -C target-cpu=help), providing just -C target-cpu=native as a shorthand for "optimise for whatever my current CPU is" would be nice.

[brson]

Should we detect the target CPU by default so you don't have to opt into arch-specific optimizations? What do other compilers do?

[thestinger]

GCC and clang have -march=architecture and allow passing -march=native to auto-detect. By default they build for the lowest common denominator.

[steveklabnik]

rustc -C target-cpu=native does in fact exist today 🎊