Build Plugin/JITLoader/GDB on all platforms.

Summary:
We currently link to this on all platforms, so don't need to re-include
it into the LLDB_USED_LIBS. Also don't need to special case building
it for every supported platform.

Reviewers: clayborg, labath

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D12756

llvm-svn: 247284