add profile-guided-optimisation target for msvc

`nmake msvc-pgo` works for me with the Visual Studio 2012 x86 command
prompt, but in the x64 command prompt, the instrumented binary crashes
with return code 0xc000007b. Not using `/ltcg:pgi` in the x64 command
prompt works fine. I do not know why this happens.

Also adds an `msvc-clean` target.