In the CMake build, convert lldbHost to be a single static library.

Previously lldbHost was built as multiple static libraries such as
lldbHostCommon, lldbHostLinux, etc.  With this patch, the CMake
build produces only a single static library, lldbHost, whose file
set is dynamically created based on the platform.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@215792 91177308-0d34-0410-b5e6-96231b3b80d8