Revert r191088, corresponding to r191823 to re-enable llvm-lto on cygming.

r191088 is "llvm/tools/Makefile: Suppress building llvm-lto on cygming, for now, probably due to LTO.dll."

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192104 91177308-0d34-0410-b5e6-96231b3b80d8