Extend the ObjectFile interface to support dynamic loading on ELF platforms.

Debuggers on ELF platforms hook into the runtime linker by monitoring a special
"rendezvous" embedded in the address space of the inferior process.  The exact
location of this structure is filled in by the runtime linker and can be
resolved by locating the DT_DEBUG entry in the processes .dynamic section.  The
new GetImageInfoAddress() method (morally equivalent to
Process::GetImageInfoAddress) provides the mechanism to locate this information.

GetEntryPoint() simply returns the address of the start symbol in the executable
if present.  It is useful to the dynamic loader plugin for ELF systems as this
is the earliest point where LLDB can break and probe the inferiors .dynamic
section and rendezvous structure.  Also, this address can be used in the
computation of the virtual base address for position independent executables.

llvm-svn: 123466