[ELF] Enable createThunks to be called more than once.

    
In preparation for supporting range extension thunks we now continually
call createThunks() until no more thunks are added. This requires us to
record the thunks we add on each pass and only merge the new ones into the
OutputSection. We also need to check if a Relocation is targeting a thunk
to prevent us from infinitely creating more thunks.
    
Differential Revision: https://reviews.llvm.org/D34034

llvm-svn: 305555