ELF: Add libcall symbols to the link when LTO is being used.

If any of our inputs are bitcode files, the LTO code generator may create
references to certain library functions that might not be explicit in the
bitcode file's symbol table. If any of those library functions are defined
in a bitcode file in an archive member, we need to arrange to use LTO to
compile those archive members by adding them to the link beforehand.

Differential Revision: https://reviews.llvm.org/D50017

llvm-svn: 338434