Move FreeBSD's thread SetName implementation to ThisThread

SetName is only used in LLDB to set a thead's own name.  Move it there
to match OS X and Windows and slightly reduce the effort in any future
HostThread/ThisThread name refactoring.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217521 91177308-0d34-0410-b5e6-96231b3b80d8