Build mold from source with -march or -mcpu in the build script about mold

[rui314]

You can do it by running the initial cmake with a CXXFLAGS like CXXFLAGS=-march=native cmake .... I did not observe any performance difference with that option on my machine though. Did you see any improvements on your machine?

I don't think we want to set it by default anyway because some people do build mold and distribute to other machines. My understanding is that most project don't compile with -march=native by default too.

[Paulo-21]

I didn't tested yet with a big software.
If you think there is no advantage to enable this option by default then we can close this issue, i can reopen it later if i see a difference linking time on my machine.
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