add the ability to call InitTargetOptionsFromCodeGenFlags from multiple objects

Summary: llvm/CodeGen/CommandFlags.h a utility function InitTargetOptionsFromCodeGenFlags which is used to set target options from flags based on the command line. The command line flags are stored in globals defined in the same file, and including the file in multiple places causes the globals to be defined multiple times, leading to linker errors. This change adds a single place in lld where these globals are defined and exports only the utility function. This makes it possible to call InitTargetOptionsFromCodeGenFlags from multiple places in lld, which a follow-up change will do.

Reviewers: davide, ruiu

Reviewed By: davide, ruiu

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D29058

llvm-svn: 293965