fix lldb-gdbserver and lldb-platform linking on RHEL 6

This if fix for bug 23774.

Trunk CMake files were changed and fix should be applied to
tools/lldb-server/CMakeLists.txt.

Patch by: Eugene Zelenko


git-svn-id: https://llvm.org/svn/llvm-project/lldb/branches/release_36@240421 91177308-0d34-0410-b5e6-96231b3b80d8