Don't lookup an object symbol name in the module.

Instead, walk the obj symbol list in parallel to find the GV. This shouldn't
change anything on ELF where global symbols are not mangled, but it is a step
toward supporting other object formats.

Gold itself is ELF only, but bfd ld supports COFF and the logic in the gold
plugin could be reused on lld.

llvm-svn: 223780