[drlvm][build] Enable /O2 for VM building with MSVC

[Aleksey Shipilev]

Hi,

I have filed HARMONY-5095 [1] as I have noticed a small boost on my GC
microbenchmarks.
The issue is trivial and can be committed with minor effort.

Thanks,
Aleksey,
ESSD, Intel

[1] https://issues.apache.org/jira/browse/HARMONY-5095