Use lld as linker and enable --icf=safe on all targets

[NobodyXu]

From #719 (comment)

[NobodyXu]

@passcod BTW, is there anyway to install lld on windows?
On ubuntu there's apt, on macos there seems to have homebrew pre-installed, but on windows ... totally not sure.
I can certainly try installing homebrew, but not sure how well it runs on windows.

And I checked actions/runner-images for installed softwares, it only mentions clang not lld.

Our CI also failed due to ld not found.

[passcod]

Windows has chocolatey, it might be in there

[NobodyXu]

Windows has chocolatey, it might be in there

Thanks, I just checked the list and chocolatey is supported.

[passcod]

Here: https://community.chocolatey.org/packages/llvm
Try

choco install llvm

[NobodyXu]

You know what, I would actually do this in #722 since the CI has failed again...

[NobodyXu]

This issue is now blocked on #725 as doing this with cross is just too complicated, but #725 is now blocked on upstream.

[NobodyXu]

I've found a solution to this:

There's actually a flag called -Zgcc-ld=lld that uses rust-lld bundled with rustup, which will save us the hazard of installing lld and configure it in cross-compilation.

Also, there's ongoing effort to stablise it (and this is in fact how I learn about this flag in the first place): rust-lang/compiler-team#510