[lld] [ELF] Change Ctx::target to unique_ptr (PR #111260)

[Peter Smith via llvm-commits]

smithp35 wrote:

I think I've worked out what could be causing the sanitizer build failure. With a static TargetInfo all fields are initialised to 0 so `std::array<uint8_t, 4> trapInstr;` will get initialised to 0x0. However changing to using new means that this could be unitialised.

https://github.com/llvm/llvm-project/pull/111260