[buildbot] Adjust gemmlowp_INSTALL_LIBDIR based on CMake's choices

[DataCorrupted]

Some of our servers default installation to lib64 instead of lib. This patch asks CMakeCache which one did it use.

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[petrhosek]

Rather than relying on the the defaults inferred by the GNUInstallDirs module and reading those out of the CMakeCache.txt file, I think it'd be cleaner to explicitly set CMAKE_INSTALL_BINDIR, CMAKE_INSTALL_LIBDIR and CMAKE_INSTALL_INCLUDEDIR .

[DataCorrupted]

explicitly set CMAKE_INSTALL_LIBDIR

It seems other repos are already doing this, I'll just follow suit and set it to lib instead.