Clean up building & linking of Process/elf-core.

Summary:
* cmake/LLDBDependencies.cmake: elf-core is already included
  globally in LLDB_USED_LIBS, so it doesn't need to be re-added
  on individual platforms.

* lib/Makefile: elf-core is linked on each platform, so move it
  to the global list of used libraries.

* source/Plugins/Makefile: elf-core is built on each platform, so
  move it to the global list of things to build.

Reviewers: clayborg, labath

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12762

llvm-svn: 247366