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

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

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/98cca0e5fcb486153ca55f981f8f65ba6da5adea

[Mark Mentovai, 2015-03-16 16:44:33]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Robert Sesek, 2015-03-16 17:35:01]

LGTM

[Mark Mentovai, 2015-03-16 17:41:53]

Committed patchset #1 (id:1) manually as
98cca0e5fcb486153ca55f981f8f65ba6da5adea (presubmit successful).