Fix codesigning of MacOSX debugserver when built with cmake.

This patch fixes the codesigning of debugserver on OSX when built with
cmake.  Without this you get this error when debugging:
    error: process launch failed: unable to locate debugserver

Note: you also need to set LLDB_DEBUGSERVER_PATH to point to your built debugserver.
e.g. export LLDB_DEBUGSERVER_PATH=`pwd`/bin/debugserver

Change by dawn@burble.org.

Tested on MacOSX 10.9.5 and Xcode 6.1 Beta using cmake/ninja.
Verified no build break on Linux Ubuntu cmake/ninja and Xcode 6.1 canonical build.