Use -mmacosx-version-min=10.6 uniformly
Use -mmacosx-version-min=10.6 uniformly

Both the Chromium build and the Gyp-based standalone NaCl build
use -mmacosx-version-min=10.6 uniformly and this works on both
10.7 and 10.8.  The SCons build should follow suit.

This also works around an incompatibility with the 10.5 ABI in
the ASan runtime build that induces a false test failure.  (The
'struct dirent' type you get changes between 10.5 and 10.6 ABIs,
and recently-built ASan interceptors for 'readdir' et al are only
compatible with the 10.6 ABI so they can yield false positives
when the callers actually use the 10.5 ABI.  This breaks the
nacl_host_dir test.)

BUG= none
TEST= trybots
R=mseaborn@chromium.org, bradnelson@google.com

Review URL: https://codereview.chromium.org/34503004

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@12288 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client