MSVC 2010 Projects: Improve Optimization A Bit

As the MSVC 2010 x64 (and later) compiler/linker do not have the
optimization bug that prevents g-ir-compiler.exe from being built (due to
a compiler/linker hang) under /O2 (max speed) (or /Ox, complete
optimization), use /O2 for the optimization setting, like the other parts
of g-i under Visual Studio 2010.