[lld][ELF] Handle multiple weak symbols

This handles multiple weak symbols which appear back to back. This fix is needed
which otherwise will lead to symbols getting initialized to arbitrary values.

There was a constructor/destructor test that really triggered this to be fixed
on X86_64.

Adds a test.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@190658 91177308-0d34-0410-b5e6-96231b3b80d8