[PATCH][GCC 4.9][PR sanitizer/70474] Fix ASan failures on new Darwin platforms (OS X 10.11+, iOS 9.0+).

[Maxim Ostapenko]

Hi,

this is adapted patch from 
https://gcc.gnu.org/ml/gcc-patches/2016-01/msg00803.html that fixes 
exactly the same issue (ASan fails on newer Darwin platforms) for GCC 4.9.4.
Regtested and bootstrapped on x86_64-unknown-linux-gnu by myself and on 
x86_64-apple-darwin15.2.0 by Dominique, OK for gcc-4_9-branch?

-Maxim
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