Clean up building & linking of Process/elf-core.
- 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
Differential Revision: http://reviews.llvm.org/D12762