[PECOFF][Driver] Prefer "/" over "-" as the indicator of the option.

This patch does not change the behavior of LLD, but changes the output of the
help text. We want to show the help text with Windows style indicator rather
than Unix style indicator.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@187544 91177308-0d34-0410-b5e6-96231b3b80d8