[ELF] - Recommit r309252 "[ELF] - Fix missing relocation when linking...

[ELF] - Recommit r309252 "[ELF] - Fix missing relocation when linking executable with --unresolved-symbols=ignore-all"

With fix for undefined weak symbols in executable.

Original commit message:
This is PR32112. Previously when we linked executable with 
--unresolved-symbols=ignore-all and undefined symbols, like:

_start:
callq und@PLT

we did not create relocations, though it looks in that case
we should delegate handling of such symbols to runtime linker,
hence should emit them. Patch fixes that.

Differential revision: https://reviews.llvm.org/D35724

llvm-svn: 309796