Fix header file references to the ::llvm::ELF namespace.

We have header files which define the namespace ::art::llvm
and if they are included before the header files fixed here,
references to llvm::ELF from ::art pick up ::art::llvm
instead of ::llvm and fail to find ELF, causing an error.
Therefore we add the leading "::".

Change-Id: Ib02c9317d39cde5331dd67463013643cbc79ee1f