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

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 thepowersgang#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

src/trans/target.cpp

@@ -541,11 +541,21 @@ namespace
                 ARCH_X86_64
                 };
         }
+        else if(target_name == "i686-apple-darwin")
+        {
+            // NOTE: OSX uses Mach-O binaries, which don't fully support the defaults used for GNU targets
+            // The first 32bit Intel Mac was Core Solo aka yonah. It allows to use `-mtune=yonah` like Rust.
+            return TargetSpec {
+                "unix", "macos", "gnu", {CodegenMode::Gnu11, false, "x86_64-apple-darwin", {"-mtune=yonah"}, {}},
+                ARCH_X86_64
+                };
+        }
         else if(target_name == "x86_64-apple-darwin")
         {
             // NOTE: OSX uses Mach-O binaries, which don't fully support the defaults used for GNU targets
+            // The first 64bit Intel Mac was Core Duo. It allows to use `-mtune=core2` like Rust.
             return TargetSpec {
-                "unix", "macos", "gnu", {CodegenMode::Gnu11, false, "x86_64-apple-darwin", {}, {}},
+                "unix", "macos", "gnu", {CodegenMode::Gnu11, false, "x86_64-apple-darwin", {"-mtune=core2"}, {}},
                 ARCH_X86_64
                 };
         }