Pass -Ctarget-cpu=native during cargo install

[Manishearth]

This lets LLVM assume that the binary will be run on a machine with all the features as the host machine (and thus optimize more e.g. with better SIMD/AVX). Which is true for cargo install, and we should pass it by default, perhaps with a flag to turn it off.

cc @rust-lang/cargo

[joshtriplett]

We can't automatically make this assumption; people do use cargo install as part of packaging systems, too.

[Nemo157]

Based on the discussion in the linked PR and @joshtriplett's comment, should this be closed as "won't fix"?

[alexcrichton]

I think so!

[joshtriplett]

We could reasonably pass this by default for build scripts, if we want to, though.

[Rudxain]

This should be closed as "not-planned", not "completed". This may be misleading

[Nemo157]

That is a github bug, there was no such thing as a close reason when this issue was closed and github assigned "completed" to all previously closed issues.

[weihanglo]

They provide a way to close it again as not planned. Lemme try.

[Rudxain]

That is a github bug, there was no such thing as a close reason when this issue was closed and github assigned "completed" to all previously closed issues.

I'm sorry for my ignorance. Thank you for clarifying this

[Rudxain]

What about a new sub-cmd? (we can bike-shed the name later)