Use _aligned_malloc instead of posix_memalign on Windows

This allows BLIS with the reference configuration to compile with MinGW (gcc 4.8.1 [from mingw-builds](http://sourceforge.net/projects/mingwbuilds/files/host-windows/releases/4.8.1/64-bit/threads-win32/seh/x64-4.8.1-release-win32-seh-rev5.7z/download), using MSYS2). The testsuite doesn't run successfully, it gets to the gemm tests then stops with an `Error 127`, see https://gist.github.com/tkelman/e1e619c2f1d2ff410068