[CMake] Add dependency on Mips target if it is available

The Mips plugins conditionally link the Mips backend, so we need to conditionally add the target as a dependency.

This resolves a bot failure from r294515.

http://lab.llvm.org:8011/builders/lldb-x86_64-ubuntu-14.04-buildserver/builds/4606/steps/build%20android/logs/stdio

llvm-svn: 294521