Add a method "GetEntryPoint" to the ObjectFile class, and implement it on...

Add a method "GetEntryPoint" to the ObjectFile class, and implement it on MachO & ELF - though the ELF implementation is probably a little weak.  Then use this method in place of directly looking for "start" in the ThreadPlanCallFunction constructor to find the stopping point for our function evaluation.

llvm-svn: 127194