[lld] [ELF] Handle and optimize x86-64 PLTOFF64 TLS sequences (PR #216263)

[Peter Smith via llvm-commits]

https://github.com/smithp35 commented:

The transformation makes sense to me, but brobably best that someone more familiar with x86_64 check over the details of the instructions. I've made some cosmetic suggestions based on my reading of the source. 

https://github.com/llvm/llvm-project/pull/216263