elf: synthesize symbols for PLT entries

    
When populating symbol tables ObjectFileELF now generates a set of synthetic
trampoline symbols.  These new symbols correspond to entries in the program
linkage table and have a (possibly mangled) name identifying the corresponding
symbol in some DSO.  These symbols will be used by the DynamicLoader loader
plugin on Linux to provide thread plans when execution flows from one DSO to
another.

llvm-svn: 128550