Enforce -mtune=core2 and -mtune=yonah on macOS

[catap]

I've used the same cpu which is used by Rust: core2 for x86_64[1] and yonah for i686[2]. I also used mtine instead of mcpu because it was deprecated at gcc since version 5[3].

It closes #214

I also added missed i686-apple-darwin target.

Footnotes:
[1] https://github.com/rust-lang/rust/blob/1.54.0/compiler/rustc_target/src/spec/x86_64_apple_darwin.rs
[2] https://github.com/rust-lang/rust/blob/1.54.0/compiler/rustc_target/src/spec/i686_apple_darwin.rs
[3] gcc-mirror/gcc@9d913bb

[bjorn3]

-mtune merely optimizes for better performance on a specific cpu while still allowing it to run on older systems, this means it doesn't enable target features supported by the specified cpu like xchg16b support. You should probably use -march instead.

[catap]

@bjorn3 very good catch. Let me fix it.