Fix Windows build break introduced in r203035.

Add '#if defined(__linux__) || defined(__FreeBSD__)' around JITLoaderGDB
and ProcessElfCore, which are only built on Linux and FreeBSD.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@203107 91177308-0d34-0410-b5e6-96231b3b80d8