[PECOFF] Add /HighEntropyVA.

This is yet another command line flag to set a bit in
DLLCharacteristics. Default on Win64 is "on".

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@216414 91177308-0d34-0410-b5e6-96231b3b80d8