[llvm-dlltool] Implement the --no-leading-underscore option

This requires being able to opt out from adding the leading underscores
in COFFModuleDefinition. Normally it is added automatically for I386
type targets. We could either move the decision entirely to all
callers, letting the caller check the machine type and decide whether
underscores should be added, or keep the logic mostly as is, but allowing
opting out from the behaviour on I386.

I went with keeping the interface as is for now.

Differential Revision: https://reviews.llvm.org/D152363