Make the PLAT_ identification work properly for mingw-win64.  Problem was
that mingw64 also defines __MINGW32__, which led to the 32-bit definitions
being used in the 64-bit case.  n-i-bz.  (Bernhard.Loos@ruecker.de)


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13971 a5019735-40e9-0310-863c-91ae7b9d1cf9