Backports of Solaris 12 patches to gcc 5 branch

[Rainer Orth]

I've now backported the following two patches

        [boehm-gc] Avoid unstructured procfs on Solaris
        https://gcc.gnu.org/ml/gcc-patches/2015-08/msg01591.html

        [build] Use __cxa_atexit on Solaris 12+
        https://gcc.gnu.org/ml/gcc-patches/2015-08/msg01593.html

to the gcc 5 branch.  They've been on mainline for two weeks without any
known issues.

        Rainer

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Rainer Orth, Center for Biotechnology, Bielefeld University