[lldb] Respect previously set values of LLDB_TABLEGEN_EXE

If you set LLDB_TABLEGEN_EXE in a CMake cache file or in the CMake
invocation line, your setting isn't respected. Setting up the tablegen
for the host will overwrite the value that we set LLDB_TABLEGEN_EXE to,
which defeats the whole point of setting it in the first place.