[OMPT] Disable by default on Windows

This is broken per PR36561 and PR36574, so disable it for now until
somebody interested can take a look. OMPT can still be activated manually
by passing -DLIBOMP_OMPT_SUPPORT=ON during configuration.

Differential Revision: https://reviews.llvm.org/D50086

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@338721 91177308-0d34-0410-b5e6-96231b3b80d8