[ELF] Create atoms for progbits sections with no symbols but still have content.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@174091 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF][x86-64] Improve unknown relocation message.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@174092 91177308-0d34-0410-b5e6-96231b3b80d8