[cmake] Only attempt to install MSVC system libraries on Windows

Newer versions of CMake (I'm on 3.10, but I believe 3.9 behaves the same
way) attempt to query the system for information about the VS 2017
install. Unfortunately, this query fails on non-Windows systems:

  cmake_host_system_information does not recognize <key> VS_15_DIR

CMake isn't going to find these system libraries on non-Windows anyway
(and we were previously silencing the resultant warnings in our
cross-compilation toolchain), so it makes sense to just omit the
attempted installation entirely on non-Windows.

Differential Revision: https://reviews.llvm.org/D41220

llvm-svn: 320724