[PECOFF] Handle /machine option before handling all the other options.

The target machine type affects the meaning of other options, in particular
how to mangle symbols. So we want to handle the option first and then parse
all the other options.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@200589 91177308-0d34-0410-b5e6-96231b3b80d8