Use more specific target CPUs on Darwin

Macs don't come with anything older than a Yonah (32bit) or Core2 (64bit),
so we can default to those targets. Clang does the same.

Author: dotdash

src/librustc_back/target/i686_apple_darwin.rs

@@ -12,6 +12,7 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::apple_base::opts();
+    base.cpu = "yonah".to_string();
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/x86_64_apple_darwin.rs

@@ -12,7 +12,7 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::apple_base::opts();
-    base.cpu = "x86-64".to_string();
+    base.cpu = "core2".to_string();
     base.eliminate_frame_pointer = false;
     base.pre_link_args.push("-m64".to_string());