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

[Fangrui Song via llvm-commits]

MaskRay wrote:

A previous patch (BTI Thunk) might introduce flakiness to the test on Windows:
`C:\ws\src\lld\test\ELF\aarch64-thunk-bti.s:250:21: error: CHECK-PADS-NEXT: expected string not found in input`

but it is unrelated to this patch

https://buildkite.com/llvm-project/github-pull-requests/builds/107064#01925dc0-0d9e-4937-b04f-b35c2516f26a

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