Fix ObjectFileElf::GetEntryPointAddress()

ELF object files do not implicitly have a symbol named "start" as an entry
point.  For example, on Linux it is often named "_start", but can be trivially
set to any symbol by passing an --entry argument to the linker.

Use the ELF header to determine the entry point and resolve the associated
section based on that address.

Also, update the linux dynamic loader to call GetEntryPointAddress instead of
GetEntryPoint.

llvm-svn: 127218