compat/mac: 10.6 SDK compatibility for pre-Xcode 4.2 10.6 SDK.

Some definitions were added to the 10.6 SDK during its lifetime, and
aren’t present in older versions of the SDK. The AVX definitions are in
this group.

Apparently, not all of Chrome’s builders use the same version of this
SDK. A try bot failure was encountered for
https://codereview.chromium.org/1011503002/ in
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_compile_dbg_ng/builds/33868/steps/compile%20%28with%20patch%29/logs/stdio.

This provides definitions in compat for when they aren’t available from
the SDK.

TEST=compile
R=rsesek@chromium.org

Review URL: https://codereview.chromium.org/1001733003

compat/mac/mach/mach.h

@@ -61,6 +61,23 @@
 
 // <mach/i386/thread_status.h>
 
+// 10.6 SDK
+//
+// Earlier versions of this SDK didn’t have AVX definitions. They didn’t appear
+// until the version of the 10.6 SDK that shipped with Xcode 4.2, although
+// versions of this SDK appeared with Xcode releases as early as Xcode 3.2.
+// Similarly, the kernel didn’t handle AVX state until Mac OS X 10.6.8
+// (xnu-1504.15.3) and presumably the hardware-specific versions of Mac OS X
+// 10.6.7 intended to run on processors with AVX.
+
+#ifndef x86_AVX_STATE32
+#define x86_AVX_STATE32 16
+#endif
+
+#ifndef x86_AVX_STATE64
+#define x86_AVX_STATE64 17
+#endif
+
 // 10.8 SDK
 
 #ifndef x86_AVX_STATE