[ELF] Add local mapping symbols to ARM PLT entries

    
Mapping symbols allow a mapping symbol aware disassembler to
correctly disassemble the PLT when the code immediately prior to the
PLT is Thumb.

To implement this we add a function to add symbols with local
binding to be defined in SyntheticSymbols.

Differential Revision: https://reviews.llvm.org/D28956

llvm-svn: 293044