[LLVM17] Adapt to `ExplicitEmulatedTLS` removal.
0d333bf0e3 removed the `ExplicitEmulatedTLS` field from `TargetOptions`.
Before that commit, `TargetMachine::useEmulatedTLS()` fell back to `TheTriple.hasDefaultEmulatedTLS()` if `ExplicitEmulatedTLS` was `false`/unset.
After that commit, `TargetMachine::useEmulatedTLS()` directly returns `Options.EmulatedTLS`, and the fallback to `TheTriple.hasDefaultEmulatedTLS()` was moved to `InitTargetOptionsFromCodeGenFlags`.
Since `rustc` does not use `InitTargetOptionsFromCodeGenFlags` (AFAICT) and instead manually builds `TargetOptions`, this PR initializes `EmulatedTLS` to `TheTriple.hasDefaultEmulatedTLS()`.
(I'm not really familiar with the details of what this option does, or if there are any tests that depend on `hasDefaultEmulatedTLS` being used correctly, so this PR is mostly untested (it does compile against LLVM17, though)).
`@rustbot` label: +llvm-main
7e143e9043