[lld] [lld][ELF][X86] Add missing X86_64_TPOFF64 case in switches (PR #77208)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks for the patch. https://maskray.me/blog/2021-08-08-toolchain-testing#the-test-checks-too-little
x86-64 TLS tests are not very well organized, but you can add a test to `x86-64-tls-pie.s`  and add `llvm-objdump -d --no-show-raw-insn` to check the disassembly.

https://github.com/llvm/llvm-project/pull/77208