Introduce a separate preprocessor macro, _LIBUNWIND_USE_DLADDR, for directly...

Introduce a separate preprocessor macro, _LIBUNWIND_USE_DLADDR, for directly controlling a dependency on dladdr(). This will allow us to use libunwind without adding a libdl dependency.

Reviewers: saugustine

Subscribers: christof, chrib, cfe-commits, echristo

Differential Revision: https://reviews.llvm.org/D48733

llvm-svn: 336014